1.

Using differentials, find the approximate value of each of the following up to 3 places of decimal. (i)sqrt(25.3)""(ii)sqrt(49.5) (iii) sqrt(0.6)""(iv) (0.009)^(1/3) (v) (0.999)^(1/10)""(vi) (15)^(1/4) (vii) (26)^(1/3)""(viii) (255)^(1/4) (ix) (82)^(1/4)""(x)(401)^(1/2) (x i) (0.0037)^(1/2)""(x ii) (26.57)^(1/3) (x iii) (81.5)^(1/4)""(x iv) (3.968)^(3/2) (xv) (32.15)^(1/5

Answer»

Solution :(i) `sqrt(25.3)`
Let `f(x) = sqrtxrArrf'(x) = 1/(2sqrtx)`
Letx= 25 andh = 0.3
Now, `f(x+H) = f(x)+hf'(x)`
` rArrf(25.3) = f (25) + 0.3 f'(25)`
` rArr sqrt(25.3)=sqrt(25) + 0.3 xx 1/(2sqrt(25)= 5+ (0.3)/10`
` = 5.03`
(ii) `sqrt(49.5)`
Let`f(x) = sqrtxrArrf' (x) = 1/(2sqrtx))`
Let ` x = 49 and h = 0.5`
Now `f(x+h) = f(x) + hf'(x)`
`rArrf(49.5) = f(49) + 0.5 xx f' (49)`
` rArrsqrt(49.5) = sqrt(49)+ 0.5 xx 1/(2sqrt(49))`
` = 7 + 5/140 = 7.036`
(iii) `sqrt(0.6)`
Let `f(x)=sqrt xrArrf'(x) = 1/(2 sqrt x)`
Let ` x = 0.64 and h = - 0.04`
Now ` f(x+h) = f(x) +hf'(x)`
` rArrf(0.6) = f(0.64)-0.004 f'(0.64)`
`rArrsqrt(0.6) = sqrt(0.64)-0.04xx 1/(2 sqrt(0.64)`
` = 0.8 - (0.04)/(2 xx 0.8)`
` = 0.8 - 0.025 = 0.775`
(iv) `(0.009)^(1//3)`
Let`f(x) = x^(1/3)rArrf'(x) = 1/3x^(2/3)`
Let` x= 0.008 andh = 0.001`
Now` f(x+h) = f(x) + hf'(x)`
` rArrf(0.009)=f(0.008) +0.001xx f'(0.008)`
` rArr(0.009)^(1/3) = (0.008)^(1/3) + (0.001)/(3 xx (0.008)^(2/3))`
` = 0.2 + (0.001)/(3 xx 0.04) `
` = 0.2 + 0.008 = 0.208`
(v) `(0.999)^(1/10)`
Let ` f(x) = x ^(1/10) rArrf'(x) = 1/(10x^(9/10))`
Let= 1 and h = - `0.001`
Now` f(x+h) = f(x) + hf'(x)`
`rArrf(0.999) =f(1) - 0.001 xx f'(1)`
` rArr(0.999)^(1/10) = (1)^(1/10) - 0.001 xx 1/(10(1) ^(9/10))`
= 1-(0.001)/(10) = 1 - 0.0001`
` = 0.9999`
(vi) ` (15)^(1/4)`
Let` y = x^(1/4)rArr(dy)/(dx)=1/(4x^(3//4))`
Let` x = 16 andx+ Delta x = 15rArr Delta x = - 1`
arx = 16
` y = (16)^(1//4) = 2 `
` (dy)/(dx) = 1/ (4 (16)^(3//4)) = 1/32`
and `Delta y = (dy) / (dx) Delta x = 1/32 (-1) = - 1/32`
`:.y + Delta y = 2 - 1/32 = 63/32`
` rArr(15) ^(1//4) = 63/32*`
(vii) ` (26)^(1//3)`
Let ` f(x) = x^(1/3)rArrf'(x) = 1/(3x^(2/3))`
Letx = 27 and h = - 1
Nowf(x+h) = f(x) + hf'(x)`
` rArrf(26) = f(27) - 1 xx f'(27)`
` rArr(26)^(1//3) = (27)^(1//3) - 1/(3(27)^(2/3))`
` = 3- 1/27 = 3- 0.037 = 2.963 = 2.963.
(viii)` (255)^(1//4)`
Let `f(x) = x^(1/4)rArrf'(x) = 1/(4x^(3/4))`
Letx = 256, h = - 1
Now` f(x +h) = f(x) + hf'(x)`
`rArrf(255) = f(256) - 1 xx f'(256)`
` rArr (255)^(1//4) = (256)^(1//4 - 1/(4(256)^3/4))`
` = 4-1/(4 xx 64) = 4 - 0.004`
(ix) Let` y = x^(1//4)rArr(dy)/(dx) = 1/(4x^(3//4))`
Letx = 81andx + ` Delta x = 82rArrDelta x = 1`
atx = 81,
` y = 81^(1//4) = 3`
` (dy)/(dx) = 1/ (4(81)^(3//4)) = 1/(108)`
and` Delta y = (dy)/(dx) * Delta x = 1/(108) xx 1 = 1/(108)`
` :.y + Delta y = 3 + 1/(108) = (325)/(108)`
` rArr(82)^(1//4) = (325)/(108)` .
(x) `(401)^(1//2)`
Let` f(x) = x^(1/2) rArrf'(x) = 1/(2X^(1//2))`
Let` x= 400 and h = 1`
Now` f(x+h) = f(x) + hf'(x)`
` rArrf(401) = f(400) + 1 xx f'(400)`
` rArr(401)^(1//2) = (400)^(1//2) + 1/(2(400)^(1//2))`
` = 20 + 1/40 = 20 + 0.025 = 20. 025`
(xi)` (0.0037)^(1//2)`
Let` f(x) = sqrtxrArrf'(x) = 1/(2sqrtx))`
Let` x= 0.0036 and h = 0.0001`
Now` f(x+h)= f(x) + h * f'(x)`
` rArrf(0.0037)= f(0.0036) + 0.0001xx f'(0.0036)`
` rArrsqrt(0.0037) = sqrt(0.0036) + 0.0001 xx 1/(2sqrt(0.0036))`
` = 0.06 + (0.0001)/(2 xx 0.06)`
` =0.06 + 0.0008 = 0.0608`
(xii) ` (26.57)^(1//3)`
Let`f(x)= x^(1/3)rArrf'(x) = 1/(3x^(2/3))`
Let` x= 27andh = - 0.43`
Now` f(x+h)= f(x) + hf'(x)`
` rArrf(26.57) = f(27) - 0.43 xx f'(27)`
` = (27)^(1/3) - (0.43 xx 1)/(3 xx (27)^(2/3))`
` = 3- (0.43)/(3 xx 9) = 3 - 0.016`
` = 2.984`
(xiii) ` (81.5)^(1//4)`
Let ` f(x) = x^(1/4)rArrf'(x) = 1/(4x^(3/4))`
Letx = 81and h = `0.5`
Now` f(x+h) = f(x) + hf'(x)`
`f(81.5) = f(81) + 0.5 f'(81)`
` rArr (81.5)^(1//4) = (81)^(1//4) + (0.5)/(4(81)^(3//4))`
`=3+(0.5)/(4 xx 27) = 3 + 0.0046`
` = 3.0046`
(xiv) ` (3.968)^(3//2)`
Let ` f(x) = x^(3/2)rArrf'(x) = 3/2 x^(1//2)`
Letx = 4 andh =- 0.032
Now ` f(x+h) = f(x) + h f'(x)`
` rArrf(3.968) = f(4) - 0.032 xx f'(4)`
` rArr(3.968)^(3/2) = (4) ^(3/2) - 0.032 xx 3/2 xx (4) ^(1/2)`
` = 8- 0.048 xx 2= 8 - 0.096`
` = 7.904` .
(xv) ` (32.15)^(1//5)`
Let` f(x) = x^(1/5)rArrf'(x) = 1/(5x^(4/5))`
Let` x = 32 andh = 0.15`
Now` f(x+ h) = f(x) +hf'(x)`
` f(32.15) = f(32) + 0.15 xx f'(32)`
` rArr(32.15)^(1//5) = (32)^(1//5) + 0.15 xx 1/(5 xx (32)^(4/5))`
`= 2+(0.15)/(5 xx 16) = 2 + 0.0019`
` = 2.0019`


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