The sum of the first 30 terms of an A.P. is 1920. If the fourth term is 18, find its 11th term. 

 

 
 
 
 
 

Answers (1)
S safeer

\\ $Given sum of first 30 terms = 1920 $ \\\\ \frac{30}{2}[2 a+29 d]=1920 \\ 2 a+29 d=128 ....(i)\\ $Given 4th term = 18 $ \\ a+3 d=18 ...(ii)\\ $ Equation (i) -2 Equation (ii) $\\ 2a+ 29d - 2a - 6d = 128 - 36 \\ 23d = 92 \\ d = 4 \\ a = 6 \\ a_{11}=a+10 d \\ = 6 + 10 \times 4 \\=46 \\

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