cos²73°+cos²47°+cos73°cos47°的值是多少

cos²73°+cos²47°+cos73°cos47°的值是多少
cos²73°+cos²47°+cos73°cos47°的值是多少

cos²73°+cos²47°+cos73°cos47°的值是多少
cos²73°+cos²47°+cos73°cos47°
=1/2(cos146-1)+1/2(cos94-1)+cos73°cos47°
=1/2(cos146+cos94)+cos73°cos47°-1
=cos120*cos26+1/2(cos120+cos26)-1(和差化积,积化和差)
=-1/2*cos26-1/4+1/2*cos26-1
=-5/4