因式分解x²+x-72 -x²-6x+10 若x、y满足2x+y-4=0求xy的最大值

题目

因式分解x²+x-72 -x²-6x+10 若x、y满足2x+y-4=0求xy的最大值

答案

x²+x-72=(x+9)(x-8)-x²-6x+10=-(x²+6x-10)=-(x²+6x+9-19)=-[(x+3)²-19]=-(x+3+√19)(x+3-√19)2x+y-4=0∴y=4-2xxy=x(4-2x)=-2x²+4x=-2(x²-2x+1)+2=-2(x-1)²+2∴x=1,y=2时有...