( 1) 1×2+2×3+3×4+? + 10× 1 1=( 1/3)× 10× 1 1× 12

(2) 1×2+2×3+3×4+? +n×(n+ 1)=( 1/3)×n×(n+ 1)×(n+2)

(3) 1×2×3+2×3×4+3×4×5+? +7×8×9 =( 1/4)×7×8×9× 10

In fact, the material is more useful than your three formulas. When the three formulas are added together, the term on the right can be eliminated, and the law can be obtained. (1) The questions in (2) are all done in this way, and the questions in (3) need to be reasoned by themselves, such as:

1×2×3=( 1/4)×( 1×2×3×4-0× 1×2×3)

2×3×4=( 1/4)×(2×3×4×5- 1×2×3×4)

3×4×5=( 1/4)×(3×4×5×6-2×3×4×5)

The above three formulas add up to:

1× 2× 3+2× 3× 4+3× 4× 5 = (1/4) × (3× 4× 5× 6), and then the answer to the question (3) can be deduced.

If LZ still doesn't understand, you can ask.