B约等于40度53分
x²>=0所以x²=-y²+3y>=0y(y-3)<=00<=y<=32x²-y²=2(-y²+3y)-y²=-3y²+6y=-3(y-1)²+30<=y<=3所以y=1,最大=3y=3,最小=-9所以-9≤2x²-y²≤3
