# Math Help - Identity help.

1. ## Identity help.

I am ABSOLUTLY stuck on this idenity problem! Anyone please help!

Prove algebraically that the equation is an identity:

(5cosC-2sinC)^2+(2cosC+5sinC)^2=29

Please help! Thank you so much!

2. I would probably multiply it out, and use $\sin^{2}(C)+\cos^{2}(C)=1$ a few times.

3. Originally Posted by wiseguy
I am ABSOLUTLY stuck on this idenity problem! Anyone please help!

Please help! Thank you so much!
Hi wiseguy,

$(5 \cos C - 2 \sin C)^2+(2 \cos C+5 \sin C)^2=29$

$25\cos^2 C-20 \cos C \sin C+4 \sin^2 C + (4\cos^2 C+20 \cos C \sin C+ 25 \sin^2 C)=29$

$29 \cos^2 C+29 \sin^2 C=29$

$29(\cos^2 C+\sin^2 C)=29$

$29(1)=29$

$29=29$

4. Thank you!! Duh to me! alskjd;f