Sin^2二十度+cos^2八十度+sin50度cos80度-3／4 求值'过程写下'谢了'''

Sin^2二十度+cos^2八十度+sin50度cos80度-3／4=sin²20°+cos²80°+sin50cos80°-3/4=(1-cos40°)/2+(1+cos160°)/2+(1/2)(sin130°-sin30°)-3/4=(cos160°-cos40°)/2+(1/2)sin130°=-(cos20°+cos40°)/...