SOLUTION: the sum of the fourth and twelfth term of an A.P is 120.what is the sum of the first 15 terms of the A.p

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Question 843209: the sum of the fourth and twelfth term of an A.P is 120.what is the sum of the first 15 terms of the A.p
Answer by richwmiller(17219) About Me  (Show Source):
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an = a + (n - 1)d
a4 = a + 3d
a12 = a + 11d
120=2a+14d
S = ½(2a + (n-1)d)n
S = ½(2a + (14)d)15
S = ½(120)15
S=900