# Thread: Identity Crisis!

1. ## Identity Crisis!

Hi, Could someone please help me prove this identity?

sinA + sin9A = 2.sin5A.cos4A

Thanks, your help would be very much appreciated!

2. Originally Posted by sammysparrow
Hi, Could someone please help me prove this identity?

sinA + sin9A = 2.sin5A.cos4A

Thanks, your help would be very much appreciated!
Use the trigonometric identity

$\displaystyle\ SinA+SinB=2Sin\left(\frac{A+B}{2}\right)Cos\left(\ frac{A-B}{2}\right)$

with $B=9A$

coupled with $Cos(-C)=CosC$

3. Thanks Archie Meade!