(sin40°×sin130°)÷(cos^4 175°- sin^4 175)=sin40*sin(180-50)/[(sin175)^2+(cos175)^2][(cos175)^2-(sin175)^2]=sin40sin50/[(cos5)^2-(sin5)^2]=sin40sin50/cos(5+5)=sin40cos40/cos10=1/2sin80/sin80=1/2