# LHS = cos10cos20 + sin45cos145 + sin55cos245 #
# = 1 / 2 2cos10cos20 + 2sin45cos145 + 2sin55cos245
sin (145-45) + sin (245 + 55) -sin (245-55) # 1 / 2 cos (10 + 20) + cos (20-10) + sin
# = 1 / 2 cos30 + cos10cancel (+ sin190) -sin100 + sin300cancel (-sin190) #
# = 1 / 2 sin (90-30) + cos10-sin (90 + 10) + sin (360-60)
# = 1 / 2 취소 (sin60) 취소 (+ cos10) 취소 (-cos10) 취소 (-sin60) #
# = 1 / 2 * 0 = 0 = RHS #
