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I'll try to answer the question, "If the 5th term of a geometric progression is 2, then the product of its FIRST 9 terms is --?"

Given the first term is A and the ratio is r, then the progression starts out...

A, Ar, Ar^2, Ar^3, Ar^4, ...

So the 5th term is Ar^4, which equals 2.

The series continues...

Ar^5, Ar^6, Ar^7, Ar^8, ...

Ar^8 is the 9th term.

The product P of all 9 terms is therefore:

P = A * Ar * Ar^2 *...*Ar^8

Collect all the A's

P = (A^9)*(1 * r * r^2 ...* r^8)

P = A^9 * r^(0+1+2+...+8)

There's a formula for the sum of the first n integers (n/2)(n+1), or if you don't know just add it up.

1+2+...+8 = 36

Therefore

P = A^9 * r^36

Since 36 is a multiple of 9, you can simplify:

P = (Ar^4)^9

Still with me? Remember that Ar^4=2 (a given fact).

So finally

P = 2^9 = 512.

Cute problem.

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Q: Q 5th term of a GP is 2 then product of its 9 terms is?
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