Answer:
First we Check if its an AP or GP
Its an AP because the Second term Minus the First term is = to 3rd term Minus Second term
Since it said "Sum"
The formula we should think Of is
Sn = n/2[2a + (n-1)d]
n=100
a(First term) =100
Common difference 'd' = Second term - First term or Third term - Second term
So
d= 98-100 or 96-98
d=-2.
Applying the Formula...
Sn = 100/2 [2(100) + (100-1)(2)]
Sn = 50[200 + (99)(2)]
Sn= 50[200 + 198]
Sn = 50[398]
Sn = 19,900.
