(-x+2x²+5)+(4x²-3-6x)等于多少
=-x+2x²+5+4x²-3-6x
=6x²-7x+2;
问题补充：
还有(3a²-ab+7)-(-4a²+2ab+7)
=3a²-ab+7+4a²-2ab-7
=7a²-3ab;
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