已知sin(a+π/4)=4/5,且π/4

问题描述:

已知sin(a+π/4)=4/5,且π/4

cos(a+π/4)=-3/5
sin(a+π/4)=4/5
cosa
=cos(a+π/4-π/4)
=cos(a+π/4)cosπ/4+sin(a+π/4)sinπ/4
=-3/5*√2/2+4/5*√2/2
=√2/2(1/5)
=√2/10

由π/4