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laila [671]
3 years ago
13

the first term of an arithmetic sequence is 2 and the sum of the first 15 terms is 292.5. Find the common difference

Mathematics
1 answer:
adelina 88 [10]3 years ago
8 0

Given:

First term of an arithmetic sequence is 2.

Sum of first 15 terms = 292.5

To find:

The common difference.

Solution:

We have,

First term: a=2

Sum of first 15 terms: S_{15}=292.5

The formula of sum of first n terms of an AP is

S_n=\dfrac{n}{2}[2a+(n-1)d]

Where, a is first term and d is common difference.

Putting S_{15}=292.5, n=15 and a=2 in the above formula, we get

292.5=\dfrac{15}{2}[2(2)+(15-1)d]

292.5=\dfrac{15}{2}[4+14d]

292.5=15[2+7d]

Divide both sides by 15.

\dfrac{292.5}{15}=2+7d

19.5=2+7d

19.5-2=7d

17.5=7d

Dividing both sides by 7, we get

\dfrac{17.5}{7}=d

2.5=d

Therefore, the common difference is 2.5.

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