✔ 最佳答案
令P(3cosA,3sinA)在圓上
xy+x+y=9cosAsinA+3cosA+3sinA=9/2(sin2A)+3(sinA+cosA)
令sinA+cosA=k
k^2=1+2sin2A
k^2-1=2sin2A
9/2(sin2A)+3(sinA+cosA)
=9/4(k^2-1)+3k
=9/4(k^2)+3k-9/4 -根號2<=k<=根號2
k=-2/3,有最小值 -13/4
當k=根號2,有最大值 9/4+3根號2
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2010-01-12 07:21:46 補充:
昨夜筆誤,更正如下
令P(3cosA,3sinA)在圓上
xy+x+y=9cosAsinA+3cosA+3sinA=9/2(sin2A)+3(sinA+cosA)
令sinA+cosA=k
k^2=1+sin2A
k^2-1=sin2A
令f(k)=9/2(sin2A)+3(sinA+cosA)
=9/2(k^2-1)+3k
=9/2(k^2)+3k-9/4 -根號2<=k<=根號2
f(-1/3)=-4-1=-5 ,當k=-1/3,有最小值 -5
f(根號2)=9/2+3根號2,當k=根號2,有最大值 9/2+3根號2