>>651
∬D　1/√(x^2+y^2) dxdy
= ∫[x=0,1]dx∫[y=0,x] 1/√(x^2+y^2) dy
= ∫[x=0,1]dx [log{y+√(x^2+y^2)}][y=0,x]
= ∫[x=0,1] log(√2 +1) dx
= log(√2 +1)