高一数学三角函数式化简

sin20°cos50°=cos70cos50
=cos(50+20)cos50
=0.5[cos120+cos(-20)]
=-0.25+0.5cos20
sin20+cos50=cos(50+20)+cos50
=2cos60cos(-10)
=cos10
原式=（cos70+cos50）^2-cos70cos50
=cos^10+0.25-0.5cos20
=0.5(1+cos20)+0.25-0.5cos20
=0.75
写的有点繁,希望能看懂