Thread: Obtuse Angle and Sine

Hello!

This is a question I couldn't solve.

If A is an obtuse angle in a triangle and sin A = 5/13, calculate the exact value of sin 2A.

I would appreciate your help,
Amine

Originally Posted by Aminekhadir

Hello!

This is a question I couldn't solve.

If A is an obtuse angle in a triangle and sin A = 5/13, calculate the exact value of sin 2A.

I would appreciate your help,
Amine

HI

A would be in the second quadrant since sin A is positive and its obtuse .

Then by phythagoras , the other side would be 12

so cos A = - 12/13 (negative because cos is negative in 2nd qua)

sin 2A = 2 sin A cos A=2(5/13)(-12/13)