Evaluate $ \displaystyle\iint\limits_R \sin{(\frac{x+y}{2})}\cos{(\frac{x-y}{2})}dA$ where R is the triangle with vertices (0,0), (2,0) and (1,1). (Hint: Use the change of variables u = (x+ y)/2, v = (x− y)/2.) My attempt to answer: But when we plot sin((x + y)/2)*cos((x-y)/2), we get the below graph: