（7-14sina)/cos2a=3
7-14sina=3cos2a
7-14sina=3(1-2sin²a)
3sin²a-7sina+2=0
(3sina-1)(sina-2)=0
∴sina=1/3
a是第一象限角
∴cosa=2√2/3
tana=1/2√2=√2/4
tan(a+π/4)
=(tana+tanπ/4)/(1-tana*tanπ/4)
=(tana+1)/(1-tana)
=(√2/4+1)/(1-√2/4)
=(9+4√2)/7
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