去括号：a-（b-c)= -(9a+b)-3c= 2a-[3b-(c+d)]= -[-（x+y)]-｛-[-（a-b)]｝=

a-（b-c)
=a-b+c
-(9a+b)-3c
=-9a-b-3c
2a-[3b-(c+d)]
=2a-3b+(c+d)
=2a-3b+c+d
-[-（x+y)]-｛-[-（a-b)]｝=
=(x+y)-(a-b)
=x+y-a+b去括号，并合并同类项：7X-(5x-5y)-y=3x-[5x-(二分之一x-3）]=（2x2-3x+1)-(-5+3x+x2）=7X-(5x-5y)-y=7x-5x+5y-y=(7-5)x+(5-1)y=2x+4y3x-[5x-(二分之一x-3）]=3x-5x+x/2-3=-3x/2-3（2x2-3x+1)-(-5+3x+x2）=2x²-3x+1+5-3x-x²=x²-6x+6添括号：2a-b2+c-d3=2a+()-d3-5x3+2x2-7x+4=-（ ）+42a-b2+c-d3=2a+(-b2+c)-d3-5x3+2x2-7x+4=-（ 5x3-2x2+7x ）+4