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51.

Solve: `3tan(theta-15^(@))=tan(theta+15^(@))`

Answer» `3tan(theta-15^(@))=tan(theta+15^(@))`
`rArr 3sin(theta-15^(@))/(cos(theta-15^(@)))=(sin(theta +15^(@))cos(theta-15^(@))`
`rArr 3.{2cos(theta+15^(@))sin(theta-15^(@))}=2sin(theta+15^(@)).cos(theta-15^(@))`
`rAr 3(sin2theta-sin30^(@))=sin2theta+sin30^(@)`
`=4 xx 1/2`
`rArr sin2theta=1`
`=sinpi/2`
`therefore 2theta=npi+(-1)^(n)pi/2`
`rArr theta=(npi)/(2)+(-1)^(n)pi/4` Ans.
52.

The solution of `costheta.cos2theta.cos3theta=1/4,0 lt theta lt pi/4` is:A. `pi/8`B. `pi/6`C. `pi/9`D. `pi/12`

Answer» Correct Answer - a
53.

`tantheta+tan(pi/3+theta)+tan((2pi)/(3)+theta)=3`

Answer» `theta=1/3(npi+pi/4)`
54.

The maximum value of `5costheta+3cos(theta+pi/3)+3` is:A. 5B. 11C. 10D. None of these

Answer» Correct Answer - c
55.

Solve: `sin3alpha=4sinalpha. Sin(theta+alpha).sin(theta-alpha)` where `alpha ne n pi, n in I`

Answer» `sin3alpha=4sinalpha. Sin(theta+alpha).sin(theta-alpha)`
`rArr 3-4sin^(2)alpha=4sin^(2)theta-4sin^(2)alpha`
`rArr 4sin^(2)theta=3`
`rArr sin^(2)=3/4`
`= (sqrt(3)/(2))^(2)=sin^(2)pi/3`
`rArr theta=npi+-pi/3`. Ans.
56.

Solve: `tan4theta=-cot(pi/6+theta)`

Answer» `theta=1/3(npi+(2pi)/(3))`
57.

Prove that: `sin(-420^(@))cos390^(@)+cos(-660^(@))sin330^(@)=1`

Answer» LHS `=sin(-420^(@))cos390^(@) + cos(-660^(@))sin330^(@)`
`-sin420^(@)cos390^(@)+cos660^(@)sin330^(@)`
`=-sin(360^(@)+60^(@))cos(360^(@)+30^(@))+cos(360^(@)+300^(@))sin(360^(@)+30^(@))`
`=-sin60^(@)cos30^(@)+cos300^(@)(-sin30^(@))`
`=-sqrt(3)/2 xx sqrt(3)/2-1/2cos(360^(@))-60^(@)`
`=-3/4-1/2 cos60^(@)`
`=-3/4 -1/2 xx 1/2`
`-3/4 -1/4 =-1`
=RHS Hence proved.
58.

Find the value of: `sin75^(@)`

Answer» `sin75^(@)`
`=sin(45^(@)+30^(@))`
`=sin45^(@)cos30^(@)+cos45^(@)sin30^(@)`
`=1/sqrt(2). sqrt(3)/2+ 1/sqrt(2).1/2 = (sqrt(3)+1)/(2sqrt(2))` Ans.
59.

`tan(pi/4+theta)+tan(pi/4-theta)=4`

Answer» `theta=npi+-pi/6`
60.

Find the general value of `theta` from the equation `tantheta+tan2theta+tan3theta=tantheta*tan2theta*tan3theta.`

Answer» `tantheta+tan2theta+tan3theta=tanthetatan2theta tan3theta`
`rArr tantheta+tan2theta=-tan3theta+tanthetatan2theta tan3theta=-tan3theta(1-tanthetatan2theta)`
`rArr (tantheta+tan2theta)/(1-tanthetatan2theta)=-tan3theta`
`rArr tan(theta+2theta)=-tan3theta`
`rArr tan3theta=-tan3theta`
`rArr 2tan3theta=0`
`rArr tan3theta=0`
`rArr 3theta=npi`
`rArr theta=(npi)/(3)`. Ans.
61.

Find the values of `sin75^(@)`.

Answer» `sin75^(@)=sin(45^(@)+30^(@))`
`=sin45^(@)cos30^(@)+cos45^(@)sin30^(@)`
`=1/sqrt(2).sqrt(3)/2+1/sqrt(2).1/2=(sqrt(3)+1)/(2sqrt(2))`. Ans.
62.

`tantheta+tan2theta+sqrt(3)tanthetatan2theta=sqrt(3)`

Answer» `theta=1/3(npi+pi/4)`
63.

`(cosx-cosy)^(2)+(sinx-siny)^(2)=?`A. `4sin^(2)(x-y)/(2)`B. `4cos^(2)(x-y)/(2)`C. `4sin(x+y)/(2)sin(xy)/(2)`D. `4cos(x+y)/(2)cos(x-y)/(2)`

Answer» Correct Answer - A
64.

If `A+B=45^(@)`, then `(1+tanA)(1+tanB)=?`A. 2B. `-2`C. 1D. 0

Answer» Correct Answer - A
65.

If `2costheta=x+1/x`, then `2cos2theta=`?A. `x^(2)-1/x^(2)`B. `x^(2)+1/x^(2)`C. `x-1/x`D. None of these

Answer» Correct Answer - B
66.

Solve: `sin^(2)theta+2costheta+1/4=0`

Answer» `sin^(2)theta+2costheta+1/4=0`
`4sin^(2)theta+8costheta+1=0`
`rArr 4(1-cos^(2)theta)+8costheta+1=0`
`rArr 4cos^(2)theta-8costheta-5=0`
`rArr 4cos^(2)theta-10costheta+2costheta-5=0`
`rArr 2costheta(2costheta-5)+1(2costheta-5)=0`
`rArr (2costheta-5)(2costheta+1)=0`
`rArr 2costheta-5=0` or `2costheta+1=0`
Now `2costheta-5=0`
`rArr costheta=-1/2=-cospi/3`
`=cos(pi-pi/3)=cos(2pi)/(3)`
`therefore theta=2npi +- (2pi)/(3)` Ans.
67.

Solve: `cos^(2)theta-sinthetacostheta-1/2=0`

Answer» `cos^(2)theta-sinthetacostheta-1/2=0`
`rArr 2cos^(2)theta-2sinthetacostheta-1=0`
`rArr (2cos^(2)theta-1)=2sinthetacostheta`
`rArr cos2theta=sin2theta`
`rArr tan2theta=1`
`rArr tan2theta=tanpi/4`
`rArr 2theta=npi+pi/4`
`rArr theta=(npi)/2+pi/8` Ans.
68.

In `DeltaABC`, prove that: `a(cos^(2)C/3-cos^(2)B/2)=(b-c).cos^(2)A/2`

Answer» LHS= `a(cos^(2)C/2-cos^(2)B/2)`
`=a[(s(s-c))/(ab)-(s(s-b))/(ac)]`
`=a.s/a[(s-c)/(b)-(s-b)/( C)]`
`
69.

`sin5theta+sin3theta+sintheta=0`

Answer» `theta=(npi)/(3), theta=npi+-pi/3`
70.

Prove that: `sin5A=5sinA-20sin^(3)A+16sin^(5)A`

Answer» LHS `=sin5A=sin(3A+2A)`
`=(3sinA-4sin^(3)A)(1-2sin^(2)A)+(4cos^(3)A-3cosA)2sinAcosA`
`=(3sinA-4sin^(3)A)(1-2sin^(2)A)+(4cos^(2)A-3)2sinAcos^(2)A`
`=(3sinA-4sin^(3)A)(1-2sin^(2)A)+[4(1-sin^(2)A)-3]2sina(1-sin^(2)A`
`=(3sinA-4sin^(3)A)(1-2sin^(2)A)+(2sinA-2sin^(3)A)(1-4sin^(2)A)`
`=3sinA-6sin^(3)A-4sin^(3)A+8sin^(5)A +2sinA-8sin^(3)A-2sin^(3)A+8sin^(5)A`
`5sinA-20sin^(3)A+16sin^(5)A` = RHS Hence Proved.
71.

If arcs of same length in two circles subtend angles of `60^0a n d75^0`at their centers, find the ratios of their radii.

Answer» Let radius of first circle be `r_(1)` and radius of second circle be `r_(2)` cm.
Angle subtended by arc of first circle at center `theta_(1)=60^(@)`
then length of arc `l_(1) = r_(1)theta_(1)`
Angle subtended by arc of second circle at center `theta_(2)=75^(@)`
then length of arc `l_(2)=r_(2)theta_(2)`.
`therefore` According to the problems, length of arcs are same.
`therefore l_(1)=l_(2)`
`rArr r_(1)0_(1)= r_(2)theta_(2)`
`rArr r_(1)/r_(2)=r_(2)theta_(2)`
`rArr r_(1)/r_(2) = theta_(2)/theta_(1) = 75^(@)/60^(@)`
`=5/4 =5:4`
Therefore, ratio of radii of circles `r_(1):r_(2)=5:4` Ans.
72.

`sqrt(3)" cosec "20^(@)-sec20^(2)=?`A. 1B. 2C. 4D. None of these

Answer» Correct Answer - c
73.

Find the value of other five trigonometric function `cotx=3/4`, x lies in third quadrant.

Answer» Given that `cotx=3/4` and x lies in third quadrant.
`cotx=3/4 rArr tan x=1/(cotx)`
`rArr tanx=1/(3//4) = 1 xx 4/3=4/3`
`therefore` x lies in third quadrant so, all ratio except tan and cot will be negative.
`therefore "cosec"^(2)x = 1+(3/4)^(2) = 1+9/16=25/16`
`rArr "cosec"x = -5/4`
`rArr sinx=1/(-5/4)= 1 xx (-4/5) =-4/5`
`cosx=(cosx)/(sinx). sinx`
`=cotx. sinx`
`=(3/4). (-4/5)`
`=-12/20 = -3/5`
`secx = 1/(cosx)=1/(-3/5) = 1 xx -5/3 -5/3`
Therefore, `sinx=-4/5, cosx=-3/5`
`tanx=4/3, "cosec"x=-5/4, secx =-5/3` Ans.
74.

Find the values of trignometric functions in Questions 6 to 10. `sin765^(@)`

Answer» `sin765^(@) = sin(2 xx 360^(@) + 45^(@))`
`=sin45^(@)=1/sqrt(2)`
Therefore, `sin765^(@)=1/sqrt(2)` Ans.
75.

`sin(-11pi)/(3)`

Answer» `sin(-11pi)/(3) = -sin(11pi)/(3)`
`=-sin(2 xx 2pi -pi/3)`
`=-(-sinpi/3) = sinpi/3 = sqrt(3)/2`
Therefore, `sin(-11pi)/(3) = sqrt(3)/2`. Ans.
76.

`sin3x+sin2x-sinx=4sinxcosx/2cos(3x)/(2)`

Answer» LHS `=sin3x+sin2x-sinx`
`=sin3x+2cos(2x+x)/(2)sin(2x-x)/(2)`
`=2sin(3x)/(2)cos(3x)/(2)+2cos(3x)/(2)sinx/2`
`=2cos(3x)/(2) [ sin(3x)/(2)+sinx/2]`
`=2cos(3x)/(2)[2sin((3x)/(2)+x/2)sin(2x-x)/(2)]`
`=2cos(3x)/(2)[sin(3x)/(2)+sinx/2]`
`=2cos(3x)(2)[2sin(2x)/(2)cosx/2]`
`=2cos(3x)/(2)[2sinxcosx/2]`
`=4sinxcosx/2cos(3x)/(2)`= RHS Hence Proved.
77.

If `cotA=5/12` and A lies in 3rd quadrant, then find the values of five other trignometric ratios.

Answer» `cotA = 5/12`
`rArr tanA=1/(cotA) = 12/15`.
Now, `"cosec^(2)A=1+cot^(2)A`
`=1+(5/12)^(2)=169/144`
`rArr "cosec"A = -13/12`
`therefore` cosecA is negative in 3rd quadrant
sinA`=1/("cosec"A)=-12/13`
`cosA=(cosA)/(sinA).sinA`
`=cotA. sinA`
`=5/12(-12/13)=-5/13`
`secA=1/(cosA)=-13/5`
78.

Find the values of the following : i) `tan ((19pi)/3)` ii) `sec((-22pi)/3)`

Answer» `tan (19pi)/(3) = tan(19 xx 180^(@))/(3)`
`tan(1140^(@))`
`tan(3xx 360^(@) + 60^(@))=sqrt(3)` Ans.
79.

Evaluate: `tan (13pi)/(12)`

Answer» `tan(13pi)/(12)=tan(pi+pi/12)`
`=tanpi/12=tan(pi/3-pi/4)`
`=(tanpi/3-tanpi/4)/(1+tanpi/3tanpi/4)=(sqrt(3)-1)/(1+sqrt(3))` Ans.
80.

`sinpi/10+sin(13pi)/(10)=?`A. 1B. `1/2`C. `-1/2`D. None of these

Answer» Correct Answer - C
81.

The interior angles of a polygon are in A.P. The smallest anlge is `((2pi)/(3))^(c )` and common difference is `5^(@)`. Find the number of sides in the polygon.

Answer» Correct Answer - 9
82.

If tan A `=a/b`, then find the value of `("a sin A- b cos A")/("a sin A + b cos A")`.

Answer» tan A = `a/b`
Now `("a sinA-b cosA")/("a sin A+bcosA")=((a sinA)/(cosA)-b)/((a sinA)/(cosA)+b)`
(Divide `N^( r)` and `D^(r )` by cos A)
`=("a tanA-b")/("a tan A+b")= (a.a/b-b)/(a.a/b+b)`
`=((a^(2)-b^(2))/b)/((a^(2)+b^(2))/b)= (a^(2)-b^(2))/(a^(2)+b^(2))`. Ans
83.

Find the angle between the large hand and small hand of a clock at the time `4:30`.

Answer» For small hand
Angle subtends in 12 hours `=360^(@)`
4 hrs 30 min = 4 hrs + `30/60` hrs `=9/2` hrs
Angle subtends in `9/2` hours `=9/2 xx 360^(@)/12=135^(@)`
For large hand
1 hour = angle subtends in 60 min. = `360^(@)`
`rArr` Angle subtends in 30 min. =`180^(@)`
`therefore` Angle between large hand and small hand at the time `4:30`
`=180^(@)- 135^(@)=45^(@)`.
84.

In `DeltaABC, a=x^(2)-1, b=2x+1, angleC=120^(@)`, find c.

Answer» Correct Answer - `(x^(2)+x+1)`
85.

Prove that: `sinA.sin(60^(@)+A).sin(60^(@)-A)=1/4sinA`

Answer» LHS `=sinA.sin(60^(@)+A).sin(60^(@)-A)`
`=1/2sinA.[2sin(60^(@)+A)-(60^(@)-A)]`
`1/2sinA[cos{(60^(@)+A)-(60^(@)-A)}]-cos{(60^(@)+A)+(60^(@)-A)}`
`=1/2sinA[cos2A-cos120^(@)]`
`=1/2[cos2AsinA-cos(90^(@)+30^(@)).sinA]`
`=1/4[2cos2AsinA+2sin30^(@)sinA]`
`=1/4[sin(2A+A)-sin(2A-A)=2.1/2.sinA]`
`=1/4[sin3A-sinA+sinA]`
`=14sin3A=R.H.S` Hence Proved.
86.

The sides of a triangle `ABC` are in the ratio `3:4:5`. If the perimeter of triangle `ABC` is `60,` then its lengths of sides are:

Answer» Let `A=3x, B=4x, C=5x`
but `A+B+C=180^(@)`
`rArr 3x+4x+5x=180^(2)`
`rArr 12x=180^(@)`
`rArr x=15^(@)`
`therefore A=3 xx 15^(@)=45^(@)`
`B=4 xx 15^(@)=60^(2)`
`C=5 xx 15^(2)=75^(@)`
Now sinA=`sin45^(@)=1/sqrt(2)`
`sinB=sin60^(2)=sqrt(3)/2`
`sinC=sin75^(@)=(sin45^(@)+30^(@))`
`=sin45^(2)cos30^(2)+cos45^(2)sin30^(@)`
`=1/sqrt(2) xx sqrt(3)/2 + 1/sqrt(2)xx 1/2=(sqrt(3)+1)/(2sqrt(2))`
`therefore a:b:c = sinA:sinB:sinC`
`=1/sqrt(2):sqrt(3)/2:(sqrt(3)+1)/(2sqrt(2))`
`=2:sqrt(6):(sqrt(2)+1)`. Ans.
87.

In `Delta ABC`, prove that: `(a+b+c)/(a-b+c)=cotA/2cotC/2`

Answer» RHS= `cotA/2.cotC/2`
`=sqrt(s(s-a))/(s-b)(s-c).sqrt((s(s-c))/(s-a)(s-b)`
`=sqrt(s^(2))/((s-b)^(2))=s/(s-b)=(2s)/(2s-2b)`
`=(a+b+c)/(a+b+c-2b)=(a+b+c)/(a-b+c)`= LHS Hence Proved.
88.

Prove that: `sec(45^(@)+A)sec(45^(@)-A)=2secA`

Answer» LHS= `sec(45^(@)+A).sec(45^(@)-A)`
`=2/((cos45^(@)+A)+(45^(@)-A))=2/(0+cos2A)=2/(cos2A)=2sec2A`=RHS Hence Proved.
89.

`cos(pi/4-x)cos(pi/4-y)-sin(pi/4-x)sin(pi/4-y)=sin(x+y)`

Answer» LHS. `=cos(pi/4-x) cos(pi/4-y)-sin(pi/4-x)sin(pi/4-y)`
`=cos[pi/2-(x+y)]`
`=sin(x+y)`= RHS Hence Proved.
90.

Solve `cos3x+cosx-cos2x=0`

Answer» Given equations:
`cos3x+cosx-cos2x=0`,
or `2cos(3x+x)/(2)cos(3x-x)/(2)-cos2x=0,`
or `2cos2xcosx-cos2x=0`
or `cos2x.(2cosx-1)=0`,
`rArr cos2x=0` or `2cosx-1=0`
If `cos2x=0`, then `2x=(2n+1)pi/2 rArr x=(2n+1)pi/4` where `n in z`
If `2cosx-1=0`, then `cosx=1/2 rArr cosx=cospi/3`
`rArr x=2npi+-pi/3`
Therefore, the general solution of given equation is `x=(2n+1)pi/2`.
or `2npi+- pi/3, n in Z` Ans.
91.

`sec^(2)2x=1-tan2x`

Answer» Given equation is `sec^(2)2x=1-tan2x`
or `1+tan^(2)2x=1-tan2x`
or `tan^(2)2x+tan2x=0`
or `tan2x(1+tan2x)=0`
`rArr tan2x=0` or `1+tan2x=0`
If `tan2x=0`, then `2x=npi`
`rArr x=(npi)/(2)` where `n in Z`
and `tan2x = 1=tan(pi-pi/4)`
`rArr tan2x=tan(3pi)/(4)`
`rArr 2x=npi + (3pi)/(4)`
`rArr x=(npi)/(2) + (3pi)/(8)` where `n in Z`
Therefore, the general solution of given equations is
`x=(npi)/(2)` or `(npi)/(2)+(3pi)/(8), n in Z` Ans.
92.

`(sinx+sin3x)/(cosx+cos3x) = tan2x`

Answer» LHS `=(sinx+sin3x)/(cosx+cos3x)`
`=(2sin(x+3x)/(2)cos(x-3x)/(2))/(2cos(x+3x)/(2)cos(x-3x)/(2)) = (sin(x+3x)/(2))/(cos(x+3x)/(2))`
`=(sin2x)/(cos2x)=tan2x`=RHS Hence Proved.
93.

Solve: `cos4x=cos2x`

Answer» Given equation: `cos4x=cos2x`
`rArr 4x=(2npi+-2x)`
then, taking `+` sign `4x = 2npi+2x`
or `4x-2x=2npi`
`rArr x=npi, n in Z`
or `4x+2x=2npi`
or `6x=2npi rArr x=(npi)/(3), n in Z`
Therefore, general solution of given equation is `x=npi`.
or `x=(npi)/(3), n in Z` Ans.
94.

(sin 3x + sin x) sin x + (cos 3x - cos x) cos x =0

Answer» LHS `=(sin3x+sinx)sinx+(cos3x-cosx)cosx`
`=[2sin(3x+x)/(2)cos(3x-x)/(2)]sinx+[2sin(3x+x)/(2)sin(x-3x)/(2)]cosx`
`=[2sin2xcosx]sinx+[2sin2xsini(-x)]cosx`
`=2sin2xsinx.cosx-2sin2x sinx cosx`
=0 = RHS Hence Proved.
95.

`cotxcot2x-cot2xcot3x-cot3xcotx=1`

Answer» `therefore cot3x=cot(2x+x)`
or `cot3x=(cotxcot2x-1)/(cot2x+cotx)`
or `cot3x(cot2x+cotx)=cotxcot2x-1`
or `-cotxcot2x+cot2xcot3x+cot3xcotx=-1`
or `-(cotxcot2x-cot2xcot3x-cot3xcotx)=-1`
Therefore,
`cotxcot2x-cot2xcot3x-cot3xcotx=1`.
Hence Proved.
96.

`(sinx-sin3x)/(sin^(2)x-cos^(2)x) = 2sinx`

Answer» LHS` = (sinx-sin3x)/(sin^(2)x-cos^(2)x)`
`=((2cos(x+3x))/(2)sin(x-3x)/(2))/(-(cos^(2)x-sin^(2)x))`
`=(2cos2xsin(-x))/(-cos2x)`
`=(-2cos2xsinx)/(-cos2x)`
`(-2cos2xsinx)/(-cos2x)`
=2sinx = RHS Hence Proved.
97.

`cotx=-sqrt(3)`

Answer» `cotx=-sqrt(3)=-cotpi/6`
`rArr cotx=cot(pi-pi/6)`or `cot(2pi-pi/6)`
`rArr cotx=cot(5pi)/(6)` or `cot(11pi)/(6)`
`rArr x=(5pi)/(6)` or `(11pi)/(6)`
Therefore, for the equation `cotx=-sqrt(3)`
Principle solution is `x=(5pi)/(6)` or `(11pi)/(6)` Ans. and general solution is `x=npi + (5pi)(6), n in Z`, Ans.
98.

`2cos(pi/13)cos(9pi/13)+cos(3pi/13)+cos(5pi/13)=0`

Answer» LHS
`2cospi/13 cos(9pi)/(13)+cos(3pi)/(13)+cos(5pi)/(13)=0`
`=2cospi/13 cos(9pi)/(13) + 2cos((5pi)/(13)+(3pi)/(13))/(2) cos((5pi)/(13)-(3pi)/(13))/(2)`
`=2cospi/13cos(9pi)/(13)+2cos(4pi)/(13)cospi/13`
`=2cospi/13(cos(9pi)/(13) + cos(4pi)/(13))`
`=2cospi/13[2cos((9pi)/(13)+(4pi)/(13))/(2) cos((9pi)/(13)-(4pi)/(13))/(2)]`
`=2cospi/13[(2cospi/2cos(5pi)/(26))]`
`=4cospi/13 cos(5pi)/(26) xx cospi/2`
`=4cospi/13 cos(5pi)/(26) xx 0` `(therefore cospi/2=0)`
=0 = RHS Hence Proved.
99.

`secx=2`

Answer» `secx = 2=secpi/3` or `sec(2pi-pi/3)`
`rArr x=pi/3` or `(2pi-pi/3)`
`rArr x=pi/3`or `(5pi)/(3)`
Therefore, principal solution of equation `secx=2` is `x=pi/3` or `(5pi)/(3)`.
`secx=2`
`1/(cosx)=2`
`cosx=1/2=cos60^(@)`
`cosx=cospi/3`
and general solution is `x=2npi+-pi/3, n in Z` Ans.
100.

`sinx+sin3x+sin5x=0`

Answer» Given equation:
`sinx+sin3x+sin5x=0`,
or `sin3x+(sin5x+sinx) =0`,
or `sin3x+2sin(5x+x)/(2)cos(5x-x)/(2)=0`,
or `sin3x+2sin3xcos2x=0`,
or `sin3x(1+2cos2x)=0`,
`rArr sin3x=0` or `1+2cos2x=0`,
`sin3x=0`
`3x=npi`
`x=(npi)/(3)`
If `1+2cos2x=0`
`rArr cos2x=-1/2= cos(pi-pi/3)=cos(2pi)/(3)`
`rArr x=npi+-pi/3` where `n in Z`
Therefore, the general solution of given equation is
`x=(npi)/(3)` or `npi +- pi/3, n in Z`. Ans.