\(\left(a+b\right)-\left(b-a\right)+c\)
\(=\left(a+b\right)-b+a+c\)
\(=2a+c\)
\(-2b=-\left(a+b+c\right)+\left(a-b-c\right)\)
\(-2b=\left[\left(-a\right)+\left(-b\right)-\left(-c\right)\right]+\left(a-b-c\right)\)
\(-2b=\left[\left(-a\right)+\left(-b\right)-\left(-c\right)\right]+a-\left(b+c\right)\)
(-a) + a = 0 nên ta có
\(\left[\left(-b\right)-\left(-c\right)\right]-\left(b+c\right)=\left[\left(-b\right)+c\right]-\left(b+c\right)\)
\(=-2b\left(đpcm\right)\)