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# CBSE Class 11-science Answered

please solve number 7
Asked by sanjuktamukherjee165 | 24 Oct, 2021, 11:04: AM Expert Answer
Sum of first n terms of AP, Sn = 2n2 + 5n

Now choose n =1 and put in the above formula,

First term = 2+5 = 7

Now put n=2 to get the sum of first two terms = 2x4 + 5x 2= 8 + 10 = 18

This means, first term + second term = 18

But, first term =7 as calculated above

So, second term = 18-7 = 11

So common difference becomes, 11-7 = 4

So the AP becomes, 7, 11, 15, ....

nth term = a + (n - 1)d = 7 + (n-1)4 = 7 + 4n - 4 = 3 + 4n
Answered by Renu Varma | 27 Oct, 2021, 11:37: PM

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