Question

cos 20°+cos 40°+cos60°+....
cos180°=​

Answer 1

Answer:

-1

Step-by-step explanation:

cos(90-70)⁰+cos(90-50)⁰+cos(90-30)⁰+cos(90-10)⁰ +cos(90+10)⁰+cos(90+30)⁰+cos(90+50)⁰+cos(90+70)⁰ +cos(180)

cos(90-O) = sinO

cos(90+O)= -sinO

sin70⁰+sin50⁰+sin30⁰+sin10⁰-sin10⁰-sin30⁰-sin50⁰-sin70⁰+cos180⁰= cos 180⁰ = -1