Math, asked by chaithanya64, 1 year ago

anwer for 3rd question​

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Answered by arc555
0

When,

n = 1

a_{1} = 5 + 2 = 7

When,

n. = 2

a_{2} = 5 + 4 = 9

Hence,

First term (a)= 7

common \: difference(d) =  a_{2} -  a_{1} = 9 - 7 = 2

S_{n} =  \frac{n}{2} (2a + (n - 1)d \: )

S _{20} =  \frac{20}{2} (14 + 19 \times 2)

S _{20} = 10 \times (1 4 + 38) = 52 \times 10 = 520

hence, option (b) is correct.

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