(全题)已知sin(α+3pi/4)=5/13,cos(pi/4-β)=3/5,且-4/pi小于α小于pi/4,pi/4小于β小于3pi/4,求cos2(α-β)?
问题描述:
(全题)已知sin(α+3pi/4)=5/13,cos(pi/4-β)=3/5,且-4/pi小于α小于pi/4,pi/4小于β小于3pi/4,求cos2(α-β)?
答
∵sin(α+3π/4)=cos(α+π/4)=5/13
cos(π/4-β)=sin(β+π/4)=3/5
又 -π/4