Evaluate ∫∫R xy dA, where R is the region bounded by y = x, y = 3x, x = 1, and x = 2.
Correct: B
∫∫R xy dA = ∫[1,2] ∫[x,3x] xy dy dx = ∫[1,2] x [y^2 / 2] from x to 3x dx = ∫[1,2] x (9x^2/2 - x^2/2) dx = ∫[1,2] 4x^3 dx = [x^4] from 1 to 2 = 16 - 1 = 15.