First slide

Arithmetic progression

Question

A non constant A.P has common difference d and first term is (1-ad). If the sum of the first 20 terms is 20, then the value of ‘a’ is

Moderate

A

$\frac{2}{19}$
B

$\frac{19}{2}$
C

$\frac{2}{9}$
D

$\frac{9}{2}$

Solution

${sum of 20 terms =S}_{20} = \frac{20}{2} [2(1-ad)+19d] =20$
$\Rightarrow a=19/2$

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