Question

If the 5th and 8th terms of a geometric sequence are 18 and 486 respectively, find r and hence find the first term.

Answer 1

well let the first term be a and let the common ration be r..
the nth term of a G.P is given by ar^n-1 so just put in the values and solve the two equations that you'll get...

Answer 2

so is 18 the first term then?

Answer 3

No the 5th term is 18 as per the question...

just think of the first term and common ration to be hypothetical for now and put in the value of n i the respective cases..
Then solve the 2 equations...

Answer 4

would it be for example, 18r^485?

Answer 5

No!!!

see your first term is a ryt?
Common ration is r,
a GP is of the form a,ar,ar^2,ar^3....ar^n-1

so as per the ques the fifth term i.e, n = 5 will be ar^4 = 18.
There that should clear it up!!

Answer 6

I can't figure out where to place everything. I have the formulas my teacher taught me yet I am unable to allocate each number to its specific area. As such I'd like to apologize for my rustiness. Thank you for your answer.

Answer 7

Hey never mind..
Anyways never mug up the formulas you won't be able to do the tricky ones better learn how they are formed, once you do that you never forget it..
:)

Answer 8

Thanks, I appreciate it! :D

Answer 9

Your most welcome!!
anything else bugging yah??
Well if there ever is just let me know..
:)

Answer 10

Will do!