Calcolare ∫∫D xy/(x2 + y2) dxdy
D
dove D = {(x, y) ∈ ℝ2 | 1 < x2 + y2 < 4, x ≥ 0, y ≥ 0}.
R
Si ha la soluzione.
∫∫D xy/(x2 + y2) dxdy = ∫01 dy ∫√(1−x2)√(4−x2) xy/(x2 + y2) dx + ∫12 dy ∫0√(4−x2) xy/(x2 + y2) dx =
∫01 dx ∫√(1−x2)√(4−x2) xy/(x2 + y2) dy + ∫12 dx ∫0√(4−x2) xy/(x2 + y2) dy =
∫01 x/2 log(x2 + y2) |1−x24−x2 dx + ∫12 x/2 log(x2 + y2) |04−x2 dx =
∫01 x log 4 dx − ∫01 x log(x2) dx + ∫12 x log 4 dx − ∫12 x log(x2) dx =
= 1/2 log 2 − 1/2 log 2 + 3/4 = 3/4.