A is a non-zero natural number. The largest number in the following formula is () A. A÷25B. A×25C. A÷125

A. A / 25 = a × 52; B, a × 25; C, a / 125 = a × 57; because: 52 > 57 > 25, a / 25 > A / 125 > a × 25; therefore: a

It is proved that for all n belonging to natural number, there are 1 / 3 & sup2; + 1 / 5 & sup2; + 1 / 7 & sup2; + +1/（2n+1）²＜1/4

From topic 1 / 3 & sup2; + 1 / 5 & sup2; + 1 / 7 & sup2; + +1 / (2n + 1) & sup2; < 1 / 4
Each item can be written in the following form
1/（2n+1）²0
So the inequality is on the left

According to the number table 1; 1 + 3; 1 + 3 + 5; 1 + 3 + 5 + 7; we can conclude an equation containing natural number n. this equation is? To be more detailed

It can be concluded that the first term a (1) = 1, the N + 1 term a (n + 1) = a (n) + 2 * n + 1 (n = 1,2,3...) So, a (n + 1) = a (n) + 2 * n + 1a (n) = a (n-1) + 2 * (n-1) + 1a (n-1) = a (n-2) + 2 * (n-2) + 1 A (3) = a (2) + 2 * 2 + 1a (2) = a (1) + 2 * 1 + 1

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