# Math Help - Trigonometric Problem

1. ## Trigonometric Problem

Hi there,
Can anyone give me help to start off this problem please?

Triangle PQR Angle PRQ = 3x; Angle PQR = x

I have to prove that p = 4q cos x cos 2x.

Thanks
Cromlix

2. Originally Posted by cromlix
Hi there,
Can anyone give me help to start off this problem please?

Triangle PQR Angle PRQ = 3x; Angle PQR = x

I have to prove that p = 4q cos x cos 2x.

Thanks
Cromlix
law of sines ...

$\dfrac{q}{\sin{x}} = \dfrac{p}{\sin(180-4x)}$

$q\sin(180-4x) = p\sin{x}$

... taking it from this point will lead to what you want proved