Question 81308
"Find the sum of the first 50 terms of the arithmetic series with first term = 
-1 and fifth term 9."
------------
a(1) = -1
a(5) = -1 + 4d = 9
4d=9+1
d=5/2=2.5
-----------------
a(50) = -1+49d=-1+49*2.5= 121.5
-------------
S(50) = (50/2)(-1+121.5)
=25(120.5)
=3012.5 
==========
Cheers,
Stan H.