S = -( -a + b + c ) + ( -c - b - a ) - ( a - b )
S = a - b - c - c - b - a - a + b
S = - a - b - 2c
\(S=-\left(-a+b+c\right)+\left(-c-b-a\right)-\left(a-b\right)\)
\(S=a-b-c-c-b-a-a+b\)
\(S=-2c-a-b\)
\(S=-2c-\left(a+b\right)\)
\(S=-2c-2020\)
hok tốt!!