If 21th term of an ap is 25 then find the sum of first 41 terms ?

1.	If 21th term of an ap is 25 then find the sum of first 41 terms ?
Answer» an = a+ ( n -1)da21 = 25a + 20d = 25a = 25 - 20dSn = n/2 [2a + l]S41=(41/2)[2(25-20d)+(40d)]=(41/2)[50-40d+40d]=50(41/2)=1025 1025..

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