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The sum of the first four terms of an A.P. is 56. The sum of the last four terms is 112. If its first term is 11, then find the number of terms.

12.  The sum of the first four terms of an A.P. is 56. The sum of the last four terms is 112. If its first term is 11, then find the number of terms.

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Given : first term =a=11

Let AP be 11,11+d,11+2d,11+3d,...........................................11+(n-1)d

Given: The sum of the first four terms of an A.P. is 56.

11+11+d+11+2d+11+3d=56

\Rightarrow 44+6d=56

\Rightarrow 6d=56-44=12

\Rightarrow 6d=12

\Rightarrow d=2

 

Also, The sum of the last four terms is 112.

11+(n-4)d+11+(n-3)d+11+(n-2)d+11+(n-1)d=112\Rightarrow 44+(n-4)2+(n-3)2+(n-2)2+(n-1)2=112

\Rightarrow 44+2n-8+2n-6+2n-4+2n-2=112

\Rightarrow 44+8n-20=112

\Rightarrow 24+8n=112

\Rightarrow 8n=112-24

\Rightarrow 8n=88

\Rightarrow n=11

Thus, the number of terms of AP is 11.

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