Ta có :
-(-a + b + c) + ( b + c - 1)
= a - b -c + b + c -1
= ( b - b ) + ( c-c ) + a -1
= 0 + 0 + a - 1
= a - 1
( 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
Vì a- 1 = a - 1
=> -(-a+b+c)+(b+c-1)=(b-c+6)-(7-a+b)+c - đpcm