If the first term of an A.P is -11 and the 5th term is 1. what is the sum of the first eight terms ?
To find the common difference of the arithmetic progression, we subtract the first term from the fifth term:
1 - (-11) = 12
So, the common difference (d) is 12.
Now we can find the sum of the first eight terms using the formula:
Sn = n/2 * [2a + (n-1)d]
where:
Sn = sum of the first n terms
a = first term
n = number of terms
d = common difference
Plugging in the values:
Sn = 8/2 * [2*(-11) + (8-1)*12]
Sn = 4 * [-22 + 7*12]
Sn = 4 * [-22 + 84]
Sn = 4 * 62
Sn = 248
Therefore, the sum of the first eight terms of the arithmetic progression is 248.