Geometric Sequence ?????????????

Subject: Geometric Sequence ????????????? Tue Sep 16, 2008 4:08 pm

I know the formula is an=a1r n-1

but in order to do that u need a ratio(r) and a first term (a1) and i have term 3 and tem 6 they were given to me in the problem

a3= 180, a6=38,880

i need to find the ratio and the 1st term affraid

Subject: Re: Geometric Sequence ????????????? Tue Sep 16, 2008 5:01 pm

Uhh not sure if you will understand my logic or if it is what the teacher wants to see..

between a3 and a6 there are 3 multiplications all by the same value x.

Therefore 180 * x * x * x = 38880
Or 180* x^3 = 38880

x^3 = 38880/180
x^3 = 216
x = cube root of 216
x = 6

Subject: Re: Geometric Sequence ????????????? Tue Sep 16, 2008 5:24 pm

I never learned this in my Geometry class....o_O" It's quite interesting...

Subject: Re: Geometric Sequence ????????????? Wed Sep 17, 2008 1:00 am

use the formula and make two equations with two unknowns, a1 and r:

180 = a1 * r2 (squared)
a1 = 180/r2

substitute in 2nd equation:

38,880 = a1 * r5 (power 5)
38,880 = 180/r2 * r5
38,888 = 180 * r3
r3 = 38,880/180 = 216
r = 6
a1 = 180/36 = 5