不解方程组x+3y=7和3x-9y=8，求x²-9y²+4x+4的值

更新时间2019-03-30 18:26:36

x2-9x2+4x+4

=(x2+4x+4)-9y2

=(x-2)2-(3y)2

=(x-2+3y)(x-2-3y)

=(x+3y-2)(x-3y-2)  [注：x+3y=7;  x-3y=(3x-9y)/3=8/3]

=(7-2)(8/3-2)

=5X2/3=10/3

x²-9y²+4x+4

=(x-3y)(x+3y)+4(x+1)

=8/3×7+4×5

=56/3+20

=116/3

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