In a triangle ABC, BC = `sqrt(39)`, AC = 5 and AB = 7. What is the measure of the angle A?

Options

A. `(pi)/(4)`
B. `(pi)/(3)`
C. `(pi)/(2)`
D. `(pi)/(6)`

Answer

B

Explanation

Let a, b, c be the sides of `Delta`ABC and `angle A = theta`

`therefore a = sqrt(39), b = 5 and c = 7`
and `cos A = (b^(2)+c^(2)-a^(2))/(2bc)=(25+49-39)/(2 xx 5 xx 7)`
`=(1)/(2)="cos"(pi)/(3)`
`rArr A = (pi)/(3)`