去括号
-0.2(5x^2+1)=
2(-a+1/2b)=
-1/2(y-2x)
-0.2(5x^2+1)=-x^2-0.2
2(-a+1/2b)=-2a+b
-1/2(y-2x)=-1/2y+x
-1/2(y-2x)=-y/2+x