11×22+22×33+33×44+…+77×88+88×99=_.
问题描述:
11×22+22×33+33×44+…+77×88+88×99=______.
答
11×22+22×33+33×44+…+77×88+88×99,
=11×11×2+11×2×11×3+11×3×11×4+…+11×7×11×8+11×8×11×9,
=11×11×(1×2+2×3+3×4+4×5+5×6+6×7+7×8+8×9),
=121×(2+6+12+20+30+42+56+72),
=121×240,
=29040;
故答案为:29040.