Question

已知一个等比数列的第三项是-18第五项是-162,它的前5项和是多少?

Answer 1

an=a1.q^(n-1)

a3=-18

a1q^2=-18 (1)

a5=-162

a1q^4=-162 (2)

(2)/(1)

q^2 =9

q= 3 or -3

q=3 ,a1= -2

q=-3,a1= -2

an = -2.3^(n-1) or -2.(-3)^(n-1)

a1+a2+...+a5

= -2( 3^5-1)/ (3-1) or -2( (-3)^5-1)/ (-3-1)

=-242 or -122