Question

In a GP, the sum of the first 3 terms is 90 and the product is 216, find Sn and S(Infinity)

Answer 1

Answer:

Step-by-step explanation:

Let the numbers be a/r, a,ar  

a/r + a +r =19  

a/r*a*ar=219  

a^3= 216  

a=6  

a+ar+ar^2 = 19r  

6+6r+6r^2=19r  

6r^2-13r+6 =0  

6r^2-9r-4r+6=0  

3r(2r-3)-2(2r-3)=0  

 

(2r-3)(3r-2)=0  

r=3/2 OR r=2/3  

plug a & r  

when r=3/2  

a/r = 6/(3/2) =4  

a=6  

ar = 9  

4,6,9  

OR r=2/3  

a/r = 6/(2/3) =9  

a= 6  

ar = 4  

9,6,4