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the 9th term of an AP is 52 while the 12th term is 70.find the sum of its 20th term

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a+8d=52
a+11d=70
subtract ....
3d = 18
d=6

sub into the 1st equation to find a, then
sum(20) = (2//2)[2a + 19d ]

the answer is 247

The 9th term

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