cos40∘+cos80∘+cos160∘+cos240∘=
=2cos(40∘+80∘2)cos(40∘−80∘2)+cos160∘−cos(180∘+60∘)[∵cosA+cosB=2cos(A+B2)cos(A−B2)]=2cos60∘ cos(−20∘)+cos160∘−12=2×12cos20∘+cos160∘−12=−cos(180−20)∘+cos160∘−12−12