2 × 1 / 3 = 1 / 2 - 1 / 3, 3 × 1 / 4 = 1 / 3 - 1 / 4, 2 × 1 / 4 = 1 / 2 × (1 / 2 - 1 / 4) 1、 What do you find in it? 2、 Use your findings to work out the following two questions （1）1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90 （2）1/1×4+1/4×7+1/7×10+1/10×13+1/13×16

2 × 1 / 3 = 1 / 2 - 1 / 3, 3 × 1 / 4 = 1 / 3 - 1 / 4, 2 × 1 / 4 = 1 / 2 × (1 / 2 - 1 / 4) 1、 What do you find in it? 2、 Use your findings to work out the following two questions （1）1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90 （2）1/1×4+1/4×7+1/7×10+1/10×13+1/13×16

1、 1 / n(n+1) = 1/n - 1/(n+1)2、（1）= 1/(1*2) + 1/(2*3) + 1/(3*4) + 1/(4*5) + 1/(5*6) + 1/(6*7) + 1/(7*8) + 1/(8*9) + 1/(9*10)= (1-1/2) + (1/2-1/3) +…… +（1/8-1/9）+（1/9-1/10）= 1 - 1/10= 9/10（2...

The most difficult math problem in the world,
At present, a 25cm long fence is used to form a rectangular chicken house. With the help of a wall (the length of the wall is long enough), and a 1m long door is left on the side parallel to the wall. If the length of the rectangle is more than twice the width, what are its length and width?
Solve the equation of first degree with one variable. It's very difficult

If its width is x meters, its length is (2x + 1) meters
2x+(2x+1)-1=25
4x+1=25
4x=24
x=6
The length is 2 * 6 + 1 = 13 meters
A: the rectangle is 13 meters long and 6 meters wide

The most difficult math problem in the world!
One side of a square is increased by 3 cm, and the adjacent side is decreased by 3 cm. The area of the rectangle is reduced by 1 cm. The area of the square is equal to the area of the square?

The area X of a square
(x+3)(x-3)-1=xx
xx-9-1=xx
unsolvable