题目内容
(1)化简下列各式:①(5a-6b)-(2a-5b);
②5x2y-[8x-3(2x2y+3x-2)].
(2)先化简,再求值:3a2b-[2ab2-2(ab-
3 |
2 |
1 |
3 |
分析:解答时要先去括号,再合并同类项.
解答:解:(1)①(5a-6b)-(2a-5b)
=5a-6b-2a+5b
=(5-2)a+(5-6)b
=3a-b;
②5x2y-[8x-3(2x2y+3x-2)]
=5x2y-8x+3(2x2y+3x-2),
=5x2y-8x+6x2y+9x-6,
=11x2y+x-6;
(2)原式3a2b-[2ab2-2(ab-
a2b)+ab]+3ab2
=3a2b-2ab2+2(ab-
a2b)-ab+3ab2=3a2b-2ab2+2ab-3a2b-ab+3ab2=ab2+ab,
当a=3,b=-
时,
原式=ab2+ab=3×(-
)2+3×(-
)=-
.
=5a-6b-2a+5b
=(5-2)a+(5-6)b
=3a-b;
②5x2y-[8x-3(2x2y+3x-2)]
=5x2y-8x+3(2x2y+3x-2),
=5x2y-8x+6x2y+9x-6,
=11x2y+x-6;
(2)原式3a2b-[2ab2-2(ab-
3 |
2 |
=3a2b-2ab2+2(ab-
3 |
2 |
当a=3,b=-
1 |
3 |
原式=ab2+ab=3×(-
1 |
3 |
1 |
3 |
2 |
3 |
点评:去括号时,括号前面是负号,去掉符号和括号,括号里面的各项要变号.
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