7. (a) Evaluate $$\sum _ { r = 10 } ^ { 30 } ( 7 + 2 r )$$ (b) (i) Write down the formula for the sum of the first \(n\) positive integers.
(ii) Using this formula, find the sum of the integers from 100 to 200 inclusive.
(iii) Hence, find the sum of the integers between 300 and 600 inclusive which are divisible by 3 .