Question

Find the acute angle between the lines whose direction ratios are 5, 12, -13 and 3, - 4, 5.

Answer 1

`cos theta=|(a_1a_2+b_1b_2+c_1c_2)/(sqrt(a_1^2+b_1^2+c_1^2)sqrt(a_2^2+b_2^2+c_2^2))|`

`=|(15-48-65)/(sqrt(25+144+169)sqrt(9+16+25))|`

`=|-98/(13sqrt2xx5sqrt2)|`

`=|-98/(13xx5xx2)|`

`=49/65`

`theta=cos^-1 (49/65)`