不解方程组x+3y=7和3x-9y=8，求x²-9y²+4x+4的值
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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