求sin2/7π+sin3/7π-sinπ/7的值(根号7)/2,怎么算的

问题描述:

求sin2/7π+sin3/7π-sinπ/7的值
(根号7)/2,怎么算的

综上所述,2sinα/(cosα-sinα-cos3α+sin3α)=1/(sin(2α)+cos(2α))=1/(-24/25-7/25)=-25/31。最后附上和差化积公式: sina+sinb=

sin(2π/7)+sin(3π/7)-sin(π/7)=sin(5π/14-π/14)+sin(5π/14+π/14)-sinπ/7=sin5π/14cosπ/14-cos5π/14sinπ/14+sin5π/14cosπ/14+cos5π/14sinπ/14-sinπ/7=2sin5π/14cosπ/14-sinπ/7=2cos(2π/14...