We have the following expression 1 + 3 + 5 +... until the 7th term.
Each term equals the previous term plus 2.
So, for any term of number n, we have:
[tex]a_{n+1}=a_1+n\cdot r_{}[/tex]
In this case, r = 2.
The sum of all therms of an arithmetic progression with 7 terms and a diference r between two consecutive terms is:
[tex]\sum ^7_{n\mathop=1}a_1+(n-1)\cdot r[/tex]
For a1 = 1 and r = 2, we got:
[tex]\sum ^7_{n\mathop=1}2n-1_{}[/tex]
