已知派/4

问题描述:

已知派/4

∵π/4<a<3π/4,∴π/2<π/4+a<π,∴sin(π/4+a)=4/5.∵0<b<π/4,∴3π/4<3π/4+b<π,∴cos(3π/4+b)=-12/13.∴sin(a+b)=-sin[π+(a+b)]=-sin[(π/4+a)+(3π/4+b)]=-[s...