Question

find the sum 18+151/2 +13+...+(-491/2)

Answer 1

series is :: 18 ,15.5 ,13, 10.5 ,, ,,, .....-49.5

This is AP series with CD = -2.5
Findin the number of terms,,,
last term= -49.5
-49.5 = 18+(n-1)(-2.5)
-49.5= 18-2.5n+2.5
2.5n = 20.5+49.5
2.5n =70
n = 28 terms


Sum of AP of total N terms = N/2(a+l)

= 28/2(18+(-49.5))
= 14(-31.5)= -441