1+sinx=sin^2 (x/2)+2sin(x/2)cos(x/2)+cos^2 (x/2)=(sin(x/2)+cos(x/2))^2
sin(x/2)=√(1+sinx)-√(1-sinx）=|sin(x/2)+cos(x/2)|-|sin(x/2)-cos(x/2)|
x=π/2, tanx无意义
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