Mathematics
rocxely1
2016-09-29 18:15:51
Find the sum of the first five terms of the geometric series in which a2 is -12 and a5 is 768
ANSWERS
DylanMonnier
2016-09-29 20:42:24

hello :  the general term is : an = ap×q^(n-p)...... q : common ratio  n=5 and p=2  : a5 = a2×q^(5-2)                         768 = -12q^3 q^3 = 768/(-12) q^3 = - 64 but : 64 = 4^3 q^3 =(-4)^3....   -4^3 =(- 4)^3 q= - 4  calculate the first term : a1 a2 = a1 q a1 =a2/q  a1 = -12/-4 a1 = 3 the sum of the first five terms  is : S= a1 × (1- q^n)/(1- q) a1 = 3        q =-4       n =5  S = 3(1-(-4)^5)/(1-(-4)).....calculate

ADD ANSWER