Write (sin13+sin167+cos13+cos167)(sin13-sin167+cos13-cos167) in the form asinx. No calculators.
I used the calculator and got 0. But without it, I can't get the answer to equal 0. I'm up to: 4sin13cos13, which by the reduction formula, equals 4sin13sin77 but then what? ...

Question

anonymous · Answer

oh nevermind! there's the sinAcosB formula

anonymous · Answer

and sin^2A+cos^2A = 1 formula

anonymous · Answer

wait, that still doesn't get me 0

anonymous · Answer

of course it does. expand and see.

anonymous · Answer

I still don't get it.. what am I supposed to do with that formula?

anonymous · Answer

are you sure that the original expression is equal to zero?

anonymous · Answer

oh... that's rather stupid of me. i don't know how to use a calculator. i think i got it this time. by sinAcosB formula, it comes down to 2sin26, which equals the right answer.. this time. Thank you so much everyone!