*(a x 7 + a x 8 - a x 15) : ( 1 + 2 + 3 + ........ + 10)
= a x (7 + 8 – 15) : ( 1 + 2 + 3 + ........ + 10)
= (a x 0) : ( 1 + 2 + 3 + ........ + 10)
= 0 : ( 1 + 2 + 3 + ........ + 10)
= 0
A= a x 7 + a x 8 = 7 + 8 = 15 vậy sẽ bằng a x 15
A= a x 15 - a x 15
A= 0 : (1 + 2 + 3 +......+ 10)
vậy a sẽ bằng : 0