thevsum of n term of an AP is (5n^2/2+3n/2). find its 20th term.
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Sum of n terms =[ 5n²/2 + 3n/2 ]
Sum of first term = 5*1/2 + 3(1)/2 = 8/2 = 4 .
Sum of second term= 5(4)/2 + 3(2)/2 = 10+3 = 13 .
difference of a first term from second = 13 - 4 = 9 .
Common difference = 9 - 4 = 5 .
20 th term = a + ( 20 - 1 )d = a+19d = 4 + 19(5) = 99 .
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