等差数列与等比数列

	等差数列	等比数列
定义	\(a_{n+1} - a_n = d\)	\(\dfrac{a_{n+1}}{a_n}\)
通项	\(a_n=a_1+(n-1)d\)	\(a_n=a_1q^{n-1}\)
等差中项:若a，b，c成等差、比数列，则b叫做a与c的等差中项，且	\(b=\dfrac{a+c}{2}\)	\(b^2 =ac\)
等和性:若m+n=p+q，则	\(a_m+a_n=a_p+a_q\)	\(a_m*a_n=a_p*a_q\)
前n项和	\(S_n = \dfrac{n(a_1+a_n)}{2}\)	当\(q \neq 1\)时，\(S_n = \dfrac{a_1(1-q^n)}{1-q} = \dfrac{a_1-a_nq}{1-q}\)，当q = 1时，Sn = na1