The given question is 8+13+18+...... which is in arithmetic progression . And for arithmetic series, we have to use the following formula for nth term .
[tex] a_{n} = a + (n-1)d [/tex]
Here a =8 , d=5
So we get
[tex] a_{n} = 8+(n-1)5 = 5n +3 [/tex]
And the required expression for n=7 is
[tex] \sum_{n=1}^{7}(5n+3) [/tex]
