(18) (18) 15. 16. use the transformation u = x + y and v = x 2y to evaluate 3xy dA for the region R bounded by the lines x 2y = 0, x 2y = 4, x + y = 4, and x + y = 1. 2 Y2 and v = 2xy to evaluate (x 2 + y2)dx dy for the use the transformation u = x 1,x2 2-9, xy = 2, and xyz 4. region R bounded by the curves x