Find the following sum: 9 + 10 + 11 + ... + 87

Answer 1

Do you mean that the sum of those numbers EQUALS 87?

Answer 2

looks like an AS
a = 9 , d = 1 , n = ?
a + (n-1)d = 87
9 + (n-1) = 87
n = 87-8 = 79

sum(79) = (79/2)(first + last)
= (79/2)(9+87) = 3792

or

sum = sum(all of 87) - sum(all of 8)
= 87(88)/2 - 8(9)/2
= 3792