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Progression Practice Test 1

Question : 4 of 23

Marks: +1, -0

Find the sum of the series 5, 11, 17, 23 .... 65.

Solution: 👈: Video Solution

Concept:

The given series is an arithmetic progression, so we use AP formulas to find the number of terms and the sum.

Explanation:

Here, first term

a=5

, common difference

d=11-5=6

, and last term

l=65

Let the number of terms be

n

Using

t_n=a+(n-1)d

65=5+(n-1)6

60=(n-1)6

n-1=10

n=11

Now use the sum formula

S_n=\frac{n}{2}(a+l)

S_{11}=\frac{11}{2}(5+65)

S_{11}=\frac{11}{2}\times 70

S_{11}=11\times 35=385

Answer:

The sum of the series is

385

, so option D is correct.

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