Quick C2 Question.Watch

#1
A geometric series has first term 4 and common ratio r. The sum of the first three terms of the series is 7.

Show that .

My working:

.

It appears to be the same equation as the one they asked for but written differently. I don't know how they expect you to arrive at .

Can anyone see where I've gone wrong? Thanks.
0
9 years ago
#2
(Original post by Muckamuck)
A geometric series has first term 4 and common ratio r. The sum of the first three terms of the series is 7.

Show that .

My working:

.

It appears to be the same equation as the one they asked for but written differently. I don't know how they expect you to arrive at .

Can anyone see where I've gone wrong? Thanks.
share a common root
0
9 years ago
#3
If the first term is 4 then the second term is 4r and the third term is 4r^2.

Do you know what they sum to?
0
9 years ago
#4
(Original post by Muckamuck)
A geometric series has first term 4 and common ratio r. The sum of the first three terms of the series is 7.

Show that .

My working:

.

It appears to be the same equation as the one they asked for but written differently. I don't know how they expect you to arrive at .

Can anyone see where I've gone wrong? Thanks.
You don't need to do any of the formula stuff

You know (or should know) that a(r^0) is the first term, a(r^1) is the second and a(r^2) is the third, where a = 4.. so add them together and make it equal 7
0
#5
(Original post by Small123)
share a common root
Thanks.

0
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