Answers

2015-06-25T14:58:28+08:00
 An is 4, 9, 14, 19, 24, ....of the form a, a+d, a+2d, ...., A1 + (n-1)d, +..... Here, a =4, d = 5 and the sum of the 
first n terms is (n/2)[2a + (n-1)d]. For n = 25 this equals (25/2)[2(4) + (25 - 1)(5)] = (25)[4 + 60] = 1600.
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