解析：∵sin(a-π/4)=√2/2(sina-cosa)
=7√2/10
∴sina-cosa=7/5
cos2a=1-2(sina)^2=7/25
得sina=±3/5
由cos2a=7/25＞0,sina-cosa=7/5＞0
a为一,二象限角
∴sina=3/5
则cosa=-4/5
tana=-3/4
∴tan(a+π/4)=(tana+1)/(1-tana)
=(1-3/4)/(1+3/4)
=1/7
楼主误将“( ”打成“9"了