∵1/(1+sinx)=1/[sin²(x/2)+cos²(x/2)+2sin(x/2)cos(x/2)]
=1/[sin(x/2)+cos(x/2)]²
=sec²(x/2)/[1+tan(x/2)]²
∴原式=∫(0,π/4)sec²(x/2)/[1+tan(x/2)]²dx
=2∫(0,π/4)d(1+tan(x/2))/[1+tan(x/2)]²
={-2/[1+tan(x/2)]}|(0,π/4)
=2-√2
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