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Which term of the AP 21,18,15,......is …

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Which term of the AP 21,18,15,......is 81?

Posted by Sachin Gothi Patidar 2 years ago

MP Board > Class 10 > Mathematics

1 answers

Preeti Dabral 2 years ago

Here the given AP is : 21,18,15,--------
Here a=21 and d=18-21=-3,a_n=-81
{tex}\mathrm{We}\;\mathrm{know}\;\mathrm{that}\;{\mathrm a}_{\mathrm n}=\mathrm a+(\mathrm n-1)\mathrm d{/tex}
{tex}\Rightarrow{/tex} -81=21+(n-1)(-3)
{tex}\Rightarrow{/tex} -81=21-3n+3

3n = 81+21+3=105
So n = {tex}\frac{105}{3}{/tex} = 35
So, -81 is the 35^th term of this AP

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MP Board > Class 10 > Mathematics