What you're doing is finding the difference between the two Sigmas. Both of them go through values of x from 8 -> 30, but the first one also goes through values of x from 4 -> 7.
It's 40, but is there a quicker method? I had to use my calculator for that (I mean algebraically btw)
Not allowed calculators in C1
The answer to my previous question was that if you take away 8 to 30 from 4 to 30, that leaves you with 4 to 7.
You can then compute that manually (i.e ∑i=47(2x−1)=((2×4)−1)+((2times5)−1).... or use that ∑i=47(2x−1)=[br]∑i=17(2x−1)−∑i=13(2x−1) using the converse of the logic at the start of the question. You can then use a formula that you know to compute both of those terms.
Or another option if you prefer, split up the sum: ∑i=47(2x−1)=2×∑i=47x−∑i=471=2∗(4+5+6+7)−1−1−1−1