The twelfth term of alinear sequence is 47 and the sum of The first 3 terms is 21 find the sum of the first 15 terms of the sequence
a + 11 d = 47 ... 3 a + 33 d =141
3 a + 2 d = 21
subtracting equations (to eliminate a) ... 31 d = 120
find a and d
plug into the sum formula
f y i ... it seems like there may be an incorrect value in the problem