设A（5cosα,4sinα),B(5cos(α+π/2),sin(α+π/2)),即B(-5sinα,cosα)
OA=√25cosα^2+16sinα^2,OB=√25sinα^2+16cosα^2=16+9sinα^2
S=1/2√(16+9sinα^2)(16+9cosα^2)=1/2√(400+81/4sin2α^2)≤41/4