The properties of equal ratio sequence and equal difference sequence The simpler the better. Try not to use English,

The properties of equal ratio sequence and equal difference sequence The simpler the better. Try not to use English,

The general term formula of arithmetic sequence an = a1 + (n-1) d an = SN-S (n-1) (n ≥ 2) an = kn + B (k, B is a constant) the first n terms and the reverse order addition derivation of the first n terms and the formula: SN = a1 + A2 + A3 ······· + an = a1 + (a1 + D) + (a1 + 2D) + ····· + [A1 + (n-1) D] 1

Formulas and properties of arithmetic sequence and arithmetic sequence

Arithmetic sequence:
The formula: an = a1 + (n-1) d
=am+(n-m)d
A = (a + b) / 2

Three different numbers from - 1,0,1,2,3 are chosen as the coefficients of quadratic function y = ax & # 178; + BX + C
How many quadratic functions have zero points on the positive and negative half axes of X axis?
How many quadratic functions have at least one zero on the x-axis with negative semiaxis?

(1) If there is a zero point on the positive and negative half axes of the x-axis, then C / A0), so there are six kinds of C (1,1) C (1,3) P (2,2) = 1 * 3 * 2. (2) there is at least one zero point on the negative half axis of the x-axis, that is, "there is one zero point on the positive and negative half axes of the x-axis" or "there are two zero points on the half axis of the x-axis (possibly coincident)", then △ = B ^ 2-4ac