Question

show that cos80°+cos40°-cos20°=0​

Answer 1

Answer:

cos80°+cos40°-cos20°

cos80°+cos40°

cosA+cosB= 2 cos[(A+B)÷2] cos [(A-B)÷2]

cos80°+cos40°= 2cos(120/2) cos(40/2)

cos80°+cos40°= 2cos(60) cos 20

                      = 2×1/2×cos20

                      = cos20°

cos80°+cos40°-cos20°= cos20°-cos20°

                                     = 0

Hence, proved