-(-a+b+c) + (b+c-1) = (b-c+6)-(7-a+b) +c
vt : a - b - c + b + c - 1
= a + (b - b) + (c - c) - 1
= a + 0 + 0 - 1
= a - 1 (1)
vp : (b-c+6)-(7-a+b) + c
= b - c + 6 - 7 + a - b + c
= (b - b) + (c - c) + a - (7 - 6)
= 0 + 0 + a - 1
= a - 1 (2)
(1) (2) => vt = vp