(1)(sina+cosa)^2+(sina-cosa)^2=2
因为a属于(0,π/2)
所以sina+cosa>0
sina+cosa=7/5
因为sina+cosa=1/5
所以sina=4/5
cosa=3/5
(2)sin2a=2sinacosa=24/25
cos2a=2cos^2a-1=-7/25
cos(2a-π/4)
=cos2a*cosπ/4+sin2a*sinπ/4
=根号2/2(sin2a+cos2a)
=根号2/2*17/25
=17根号2/50
(1)(sina+cosa)^2+(sina-cosa)^2=2
因为a属于(0,π/2)
所以sina+cosa>0
sina+cosa=7/5
因为sina+cosa=1/5
所以sina=4/5
cosa=3/5
(2)sin2a=2sinacosa=24/25
cos2a=2cos^2a-1=-7/25
cos(2a-π/4)
=cos2a*cosπ/4+sin2a*sinπ/4
=根号2/2(sin2a+cos2a)
=根号2/2*17/25
=17根号2/50