What is the sum of one, two, three

Question

What are the sums of the numbers?

1,2,3

7,8,9

4,5,6

Answer 1

sum of 1 through 9

= 45

an = a1 +(n-1)d = the nth term, a1 = 1st term =1

Sum of an arithmetic sequence with common difference, d, =1, an=a9= 9th term = 9

Sn= (n/2)(a1+an)

S9 = (9/2)(1+9) = 4.5(10) = 45

or Guass' formula

Sn = (n)(n+1)/2

S9 = 9(10)/2 = 90/2 = 45

or maybe you meant

sum of 1+2+3 = 6 = 3(4)/2

sum of 7+8+9 = 24 = 9(10)/2 - 15 -6 = 45-21 = 24

sum of 4+5+6 = 15 = 6(7)/2 - 6 =21-6=15