# Math Help - problem find the Exact Value of cos35 sin55 + sin 35 cos 55?????

1. ## problem find the Exact Value of cos35 sin55 + sin 35 cos 55?????

i am having trouble remember how to solve this problem without a calculator any words of wisdom would be greatly appreciated

2. Originally Posted by Gamaltes
i am having trouble remember how to solve this problem without a calculator any words of wisdom would be greatly appreciated
you need this identity ...

$\sin(x+y) = \sin{x}\cos{y}+\cos{x}\sin{y}$

3. but how do i solve from there... sorry I'm really lost

4. Originally Posted by Gamaltes
i am having trouble remember how to solve this problem without a calculator any words of wisdom would be greatly appreciated
The answer ought to be 1, since

$35^o+55^o=90^o$

Therefore, viewed from within a right-angled triangle

$cos35^o=sin55^o$

$cos55^o=sin35^o$

so

$cos35^osin55^o+sin35^ocos55^o=cos^235^o+sin^235^o= sin^255^o+cos^255^0=1$

5. AAAAAAHHHH there it is.... now i remember.... thanks a lot folks