解
∵a是锐角
即a∈(0.π/2)
∴a+π/4∈(π/4,3π/4)
∴sin(a+π/4)>0
∵cos(a+π/4)=3/5——利用sin²a+cos²a=1
∴sin(a+π/4)=4/5
∴sina
=sin[(a+π/4)-π/4]
=sin(a+π/4)cosπ/4-cos(a+π/4)sinπ/4
=√2/2(4/5-3/5)
=√2/10