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Let∑k=0∞crkand∑k=0∞bvk be two convergent geometric series. If b and v are both nonzero, prove that ∑k=0∞crkbvk is a geometric series. What condition(s) must be met for this series to converge?

Short Answer

Expert verified

∑k=0∞crkbvk=cb∑k=0∞rvk and so the series is in geometric progression.

The condition for the series ∑k=0∞crkbvkto be convergent is rv<1.

Step by step solution

01

Step 1. Given Information.

∑k=0∞crkand∑k=0∞bvkbe two convergent series.

02

Step 2. Prove that the series is geometric.

The expanded form of the series ∑k=0∞crkbvkwill be:

∑k=0∞crkbvk=cb1+rv+rv2+...=cb∑k=0∞rkvk=cb∑k=0∞rvk

Therefore the series is in geometric progression.

03

Step 3. Conditions met for the series to converge.

For any geometric series to converge, its common ratio must be less than 1.

The series ∑k=0∞crkbvkis in geometric progression and its common ratio is rv.

So the condition for the series to converge is given asrv<1.

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