求下列各式的值 1.sin20°cos70°+sin10°sin50° 2.cos八分之五πcos八分之π

题目

求下列各式的值 1.sin20°cos70°+sin10°sin50° 2.cos八分之五πcos八分之π

答案

1.sin20°cos70°＋sin10°sin50° =sin20°sin20°＋(cos40°-cos60°)/2 =(1-cos40°)/2＋(cos40°-cos60°)/2 =(1-cos60°)/2=(1-1/2)/2=1/4.2.cos(5π/8+π/8)=cos5π/8cosπ/8-sin5π/8sinπ/8=-√2/2cos(5π/8...