SOLUTION: An AP has 21 terms.The sum of 10th,11th, and 12th terms is 129 and the sum of the last 3 terms is 237. Find the AP

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Question 841450: An AP has 21 terms.The sum of 10th,11th, and 12th terms is 129 and the sum of the last 3 terms is 237. Find the AP
Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
a+10d+a+11d+a+12d=129,
a+19d+a+20d+a+21d=237
a = -1, d = 4
t=-1+4n