A.3=5.6.3+6.7.3+...+30.31.3
A.3=5.6.(7-4)+6.7.(8-5)+...+30.31.(32-29)
A.3=5.6.7-4.5.6+6.7.8-5.6.7+...+30.31.32-29.30.31
A.3=(5.6.7-5.6.7)+...+(29.30.31-29.30.31)+(30.31.32-4.5.6)
A.3=0+...+0+30.31.32-4.5.6
A.3=30.31.32-4.5.6
A=30.31.32-4.5.6 /3
A=(29760-120)/3
A=29460/3
A=9880
vậy A là 9880
lưa ý dấu này/ nghĩa là chia
3.A = 5.6.(7-4) + 6.7.(8-5) + ....+30.31.(32- 29)
3.A = 5.6.7 - 4.5.6 + 6.7.8 - 5.6.7 + ...+ 30.31.32 - 29.30.31
3.A = (5.6.7 + 6.7.8 + ...+ 30.31.32) - (4.5.6 + 5.6.7 + ...+ 29.30.31)
3.A = 30.31.32 - 4.5.6 = 29 640 => A = 9 880