Formula for Finding the nth
term of an A.P. = Tn = a + (n-1) d
Using this we can calculate the the fourth term and the 12th term of this AP ..
T4 = a + 3d
T12 = a+11 d
We know that T4 + T12 = 20 a
Therefore ; a+3d + a + 11d = 20 ----> 2a + 14 d...
The question asks us to find the sum of the first 15 terms ..
Therefore using the formula for the Sum of n terms of an AP we get :S15 = n/2 [ 2a + (15-1)d]
[/b] ----> n/2 [ 2a + 14 d]
We have calculated the value for 2a + 14 d to be 20 , therefore we have S 15 = 15/2 (20) = 150 (C)
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