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1.7.5 Geometric sequences and series (A-level only)

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Question 34

A subscription-based fitness app tracks the number of new premium users each month. In the first three months after a major update, the number of new premium users, in thousands, was recorded as:

3k−28,k,k+6 3k - 28, \quad k, \quad k + 6 3k−28,k,k+6

where kkk is a constant. Given that the number of new users joining each month is modeled as a geometric series,

a.

show that k2−5k−84=0k^2 - 5k - 84 = 0k2−5k−84=0.

[3]
b.

Predict the number of new premium users joining in the 6th month.

[2]
c.

The total number of all premium users who have joined the app since the update is predicted to exceed 5 million for the first time in the NNNth month.

Calculate the value of NNN according to the model.

[4]

1.7.5 Geometric sequences and series (A-level only) Questions

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