Find the sum of first 20 terms of the arithmetic series in which 3rd term is 7 and 7th term is 2 more than three times its 3rd term.

740

Step-by-step explanation:

Let the AP be a, a+d, a+2d, a+3d, ...

The  term of AP is given by:

We are given that the third term is 7.

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Also, the seventh term is 2 more than three times the third term.

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We can put it in (1)

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⇒d=4

Now, the Sum of n terms of an AP is given by the formula:

So, Sum of first 20 terms would be:

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Thus, The Sum of first 20 terms of the AP is 740.

answer: 180

step-by-step explanation:

12-2=10

20-2=18

10x18=180