Cell formula: sn = a 1+a2+a3+...+ An.

① when q≠ 1, sn = a1(1-q n)/(1-q) or sn = (a1-an× q) ÷ (/kloc)

② when q= 1, Sn=n×a 1(q= 1).

Virus formula: (n- 1) square.

Handshake formula: 1n(n- 1).

Extended data:

Conditions for establishment:

The establishment of a quadratic equation with one variable must meet three conditions at the same time:

(1) is the whole equation, that is, both sides of the equal sign are algebraic expressions, if there is a denominator in the equation; And the unknown is on the denominator, then this equation is a fractional equation, not a quadratic equation. If there is a root sign in the equation and the unknown is within the root sign, then the equation is not a quadratic equation (it is an irrational number equation).

(2) contains only one unknown number;

③ The maximum number of unknowns is 2.

Around 300 BC, Euclid of ancient Greece (about 330-275 BC) proposed a more abstract geometric method to solve quadratic equations. Diophantine (246 ~ 330) in ancient Greece only took the positive root of a quadratic equation in the process of solving a quadratic equation with one variable. Even if both of them were positive roots, he only took one.

Baidu Encyclopedia-One-variable Quadratic Equation