1.

Find the value of 6 + 11 + 16 + 21 + ....... + 71.​

Answer»

Given ,

First term = 6

Common DIFFERENCE = 11 - 6 = 5

Last term = 71

We know that , the general formula of an AP is given by

A = a + (n-1)d

So ,

71 = 6 + (n-1)5

65/5 = (n-1)

13 = n-1

n = 14

We know that , the sum of first n TERMS of an AP is given by

\large \sf \fbox{s = \frac{n}{2}(a + l)}

Substitute the known values , we get

Sum = 14/2(6 + 71)

Sum = 7(77)

Sum = 539

Hence , 6 + 11 + 16 + 21 + ....... + 71 is equal to 539



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