(cos71°)^2+cos71°*cos49°+(cos49°)^2
=（1+cos142)/2+(cos120+cos22)/2+(1+cos98)/2
=(2+cos142+cos98+cos120+cos22)/2
=3/4+(cos142+cos98+cos22)/2
=3/4+[(cos142+cos98)+cos22]/2
=3/4+(2*cos120cos22+cos22)/2
=3/4+(-cos22+cos22)/2
=3/4
有几步是和差化积和积化和差,不知道你们省份要不要求掌握,这是经典老题了