Problem on series

[appi]

The maximum value of the sum of the series 50,46,42 ... is ??

[Math Tutor]

look at this:
arithmetic series

We can use this forumula:
Sum = 0.5 * (2a1 + d(n-1))n

d = 4
an = 50
a1 = 2

we need to know n=?
50 = 4*12 + 2 so numbers in the series are 12 + 1 = 13 or u just count the numbers

Sum = 0.5 * (2*2 + 4*(13-1))*13 = 0.5 * 52 * 13 = 26 * 13 = 338