π/4+x属于(π/4,π/2)
sin(π/4+x)>0
sin²+cos²=1
所以sin(π/4+x)=12/13
所以2sin(π/4+x)cos(π/4+x)=120/169
sin(π/2+2x)=120/169
cos(2x)=120/169
x∈（0,π/4）cos（π/4+x）=5/13 则cos2x=
π/4+x属于(π/4,π/2)
sin(π/4+x)>0
sin²+cos²=1
所以sin(π/4+x)=12/13
所以2sin(π/4+x)cos(π/4+x)=120/169
sin(π/2+2x)=120/169
cos(2x)=120/169