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1. A geometric progression has first term 20 and common ratio 0.9. Find the sum of the first 30 term. How do i do this?
2. (Original post by Chelsea12345)
A geometric progression has first term 20 and common ratio 0.9. Find the sum of the first 30 term. How do i do this?
a= 20, r = 0.9 and you have a formula for the sum of the first n terms (I believe its in the formula book,so you dont even have to learn it off, although double check this as im not sure)

its
3. (Original post by DylanJ42)
a= 20, r = 0.9 and you have a formula for the sum of the first n terms (I believe its in the formula book,so you dont even have to learn it off, although double check this as im not sure)

its

Our tip is to also learn the proof of the formula since examiners like to ask for this quite often.

It is:

Sn = a + ar + ar^(2) + .. + ar^(n-1)

Then multiply by r:

rSn = ar + ar^(2) + ... + ar^(n)

Take one from the other: Sn - rSn = a + ar^(n)

Factorise both sides: Sn (1 - r) = a(1- r^(n))

Which gives the formula after solving for Sn.

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