sin4x*sin5x=sin3x*sin8x


https://www.youtube.com/watch?v=J1QF3u41pyI&feature=youtu.be

이게 제일 깔끔하긴 함 다른 풀이는


sin5x = 2sin3x*cos4x (sin8x = 2 sin4x cos4x , sin4x~=0)
sin5x = (sin3x*cos4x + sin4x*cos3x) + (sin3xcos4x- sin4xcos3x)
sin5x = sin7x + (sin3xcos(-4x) + sin(-4x)cos3x) = sin7x - sinx
sinx = sin7x - sin5x
sinx = 2 sinx cos6x 
cos6x = 1/2 (sinx~=0) 6x = pi/3 , x = pi/18