若cos(∏/6 -α)=1/3,求cos(5∏/6+α)sin(2∏/3 -α)的值
问题描述:
若cos(∏/6 -α)=1/3,求cos(5∏/6+α)sin(2∏/3 -α)的值
答
cos(π/6 -α)=1/3cosπ/6cosα+sinπ/6sinα=1/3根号3/2cosα+1/2sinα=1/3两边平方得:3/4cos^2α+1/4sin^2α+根号3/2sinαcosα=1/91/4(cos^2α+1/4sin^2α) + 1/2cos^2α +根号3/4 sin2α = 1/91/4(cos^2...