Chứng minh rằng ? Cos10 ° cos20 ° + Sin45 ° Cos145 ° + Sin55 ° Cos245 ° = 0

# LHS = cos10cos20 + sin45cos145 + sin55cos245 #

# = 1/2 2cos10cos20 + 2sin45cos145 + 2sin55cos245 #

# = 1/2 cos (10 + 20) + cos (20-10) + sin (45 + 145) -sin (145-45) + sin (245 + 55) -sin (245-55) #

# = 1/2 cos30 + cos10cattery (+ sin190) -sin100 + sin300cattery (-sin190) #

# = 1/2 sin (90-30) + cos10-sin (90 + 10) + sin (360-60) #

# = 1/2 hủy (sin60) hủy (+ cos10) hủy (-cos10) hủy (-sin60) #

# = 1/2 * 0 = 0 = RHS #