-(-a + b + c) + (b + c - 1) = (b - c + 6) - (7 - a + b) + c
VT VP
VP = (b - c + 6) - (7 - a + b) + c VT = -(-a + b + c) + (b + c - 1)
= b - c + 6 - 7 + a - b + c = a - b - c + b + c - 1
= (b - b) + (-c + c) + a + (6 - 7) = a + (-b + b) + (-c + c) + (-1)
= 0 + 0 + a + (6 - 7) = a + 0 + 0 + (-1)
= a + (6 - 7) = a + (-1)
= a + (-1) = a - 1
= a - 1
\(\Rightarrow VT=VP\)
Vậy -(-a + b + c) + (b + c - 1) = (b - c + 6) - (7 - a + b) + c