1.

Find the angle between the following pairs of lines:(i) ` -> r=2 hat i-5 hat j+ hat k+lambda(3 hat i+2 hat j+6 hat k)`and ` -> r=7 hat i-6 hat k+mu( hat i+2 hat j+2 hat k)`(ii) ` -> r=3 hat i+ hat j-2 hat k+lambda( hat i- hat j-2 hat k)`and ` -A. `cos^(-1)((8sqrt(3))/(15))`B. `cos^(-1) ((6sqrt(2))/(5))`C. `cos^(-1)((5sqrt(3))/(8))`D. `cos^(-1)(5sqrt(2))/(6))`

Answer» Correct Answer - A
The given lines are `vec(r ) =vec(a)_(1) + lambda vec(b)_(1) " and " vec( r) vec(a)_(2) +lambda vec(b)_(2)`
`:. " cos "theta =(|vec(b)_(1).vec(b)_(2)|)/(|vec(b)_(1)||vec(b)_(2)|)=(|(hat(i)-hat(j)-2hat(k)).(3hat(i)-5hat(j) -4hat(k))|)/{{sqrt(1^(2)+(-1)^(2))+(-2)^(2)}{sqrt(3^(2)+(-5)^(2)+(-4)^(2))}}`
``=((3+5 +8))/((sqrt(6)xx sqrt(50))) =(16)/(10sqrt(3)) =(8)/(5)xx(sqrt(3))/(3)=(8)/(15) sqrt(3)`
`rArr theta = "cos"^(-1) ((8sqrt(3))/(15))`


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