sin(-π/7)和sin(-π/5)、cos4/7 π 、cos5/8 π 怎么算?

问题描述:

sin(-π/7)和sin(-π/5)、cos4/7 π 、cos5/8 π 怎么算?
如题!

sin(-π/7)
= - sin(π/7)
sin(-π/5)
= - sin(π/5)
cos(4π/7)
= - cos(3π/7)
cos(5π/8)
= - cos(3π/8)cos(4π/7)= - cos(3π/7) cos(5π/8) = - cos(3π/8) 怎么来的?cos(4π/7)= - cos(π - 4π/7)= - cos(3π/7)cos(5π/8) = - cos(π - 5π/8)= - cos(3π/8)