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Geometric Sequence Question

Can someone help me with part A

here's my working:

un = 4r^n-1

s 3 = 4 (r^3 -1) / r -1 = 7

7r - 7 = 4r^3 - 4
4r^3 - 7r + 3
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Original post by Hollymae764
Can someone help me with part A

here's my working:

un = 4r^n-1

s 3 = 4 (r^3 -1) / r -1 = 7

7r - 7 = 4r^3 - 4
4r^3 - 7r + 3

the standard geometric sequences formula for nth value is an = ar^(n-1). as you've put you know that a=4. then it says that the first three terms added up = 7 so by using the above formula
n=1, a1=4
n=2, a2 = 4r
n=3, a3 = 4r^2
as we've said the first 3 terms equal 7 so
4 + 4r + 4r^2 = 7 and if you rearrange this it gives you 4r^2 +4r -3 = 0

hope this helps!
Original post by c1student
the standard geometric sequences formula for nth value is an = ar^(n-1). as you've put you know that a=4. then it says that the first three terms added up = 7 so by using the above formula
n=1, a1=4
n=2, a2 = 4r
n=3, a3 = 4r^2
as we've said the first 3 terms equal 7 so
4 + 4r + 4r^2 = 7 and if you rearrange this it gives you 4r^2 +4r -3 = 0

hope this helps!

Thank you!

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