Bài 1: \(A=\left(\dfrac{1}{3}a-\dfrac{1}{3}b\right)-\left(a-2b\right)\)
\(=\dfrac{1}{3}a-\dfrac{1}{3}b-a+2b=-\dfrac{2}{3}a+\dfrac{5}{3}b\)
\(B=\dfrac{1}{3}a-\dfrac{1}{3}b-\left(a-b\right)\)
\(=\dfrac{1}{3}a-\dfrac{1}{3}b-a+b=-\dfrac{2}{3}a+\dfrac{2}{3}b\)
\(A+B=-\dfrac{2}{3}a+\dfrac{5}{3}b+\dfrac{-2}{3}a+\dfrac{2}{3}b=-\dfrac{4}{3}a+\dfrac{7}{3}b\)
\(A-B=-\dfrac{2}{3}a+\dfrac{5}{3}b+\dfrac{2}{3}a-\dfrac{2}{3}b=b\)
Bài 2:
\(C=x-\left[b-\left(c-a-b\right)\right]\)
\(=x-\left[b-c+a+b\right]=x-\left[2b-c+a\right]\)
=x-2b+c-a
\(D=b+\left[a-\left(c-b-a\right)\right]\)
\(=b+\left[a-c+b+a\right]\)
\(=b+2a-c+b=2a+2b-c\)
C+D
=x-2b+c-a+2a+2b-c
=a+x
C-D
=x-2b+c-a-2a-2b+c
=-3a-4b+2c+x
