考虑x>=0,y>=0,x+y=1,
设x=cos^t,y=sin^t,则
acos^t+bsin^t=(1/2)[a(1+cos2t)+b(1-cos2t)]
=(a+b)/2+[(a-b)/2]cos2t<=1,
-1<=cos2t<=1,
∴a<=1,且b<=1.
又a>=0,b>=0,
∴a,b为坐标的P(a,b)所形成的平面区域是边长为1的正方形,它的面积=1.