Find the sum of the series :4 + 8 + 12 +......... + 100. from M

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313. Find the sum of the series :

4 + 8 + 12 +......... + 100.


Let ‘b’ be the first term and ‘d’ the common difference of the A .P.
a = 4, d = 4,  a= 100 =  1
We know that,  a= a+ (n -1)d
rightwards double arrow   100 = 4 + (n - 1) d
rightwards double arrow   100 = 4 + (n - 1) 4
rightwards double arrow    4n = 100
rightwards double arrow    n = 25

And comma space space straight S subscript straight n equals straight n over 2 left square bracket straight a plus straight l right square bracket space space space left square bracket because space straight l space equals straight T subscript straight n right parenthesis right square bracket
So comma space space space space straight S subscript 25 equals 25 over 2 space space space left square bracket 4 space plus space 100 right square bracket
space space space space space space space space space space space space space space equals 25 over 2 cross times 104 equals 25 cross times 52
space space space space space space space space space space space space space space space equals space 1300
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