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Formula for Sum of n Terms of an AP

Find the sum ...

Question

Find the sum of the first 20 terms of the arithmetic series if $a_{1} = 10$ and $a_{20} = 100$ .

A

1000

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B

1100

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C

1200

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D

1300

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Solution

The correct option is B 1100
To find the sum of the first n terms of an arithmetic series use the formula,
$S_{n} = \frac{n (a_{1} + a_{n})}{2}$
$S_{20} = \frac{20 (10 + 100)}{2}$
$S_{20} = \frac{2200}{2}$
$S_{20} = 1100$

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