# CBSE Class 10-Mathematics: Chapter –5 Arithmetic Progressions Part 21

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**Question 11:**

If the sum of terms of an AP is and its term is , find the value of

**Answer:**

Put

**Question 12:**

If the sum of three numbers in AP, be and their product is , find the numbers.

**Answer:**

Let no. be

**Question 13:**

If are in AP, then prove that are in AP

**Answer:**

Given are in AP

Then

If are in AP then

From (i) and (ii),

Hence proved.

**Question 14:**

If be the sum of , and 3n terms respectively of an AP, prove that

**Answer:**

R.H.S