向量a=（sina,1）b=(cosa,2）
(1)
a·b=sinacosa+2=17/8
∴sinacosa=-1/8
∴(sina+cosa)^2
=1+2sinacosa
=1-1/4
=3/4
(sina-cosa)^2
=1-2sinacosa
=1+1/4
=5/4
∵a∈(0,π/4)
∴sina