its not equal to zero it just tends towards zero - essentially implying the trapezium rule with very very thin trapezia which is equal to the integral
They're not wrong. The notation on Edexcel's part wasn't very good and as written it is equal to zero, but people seemed to generally know what they meant.
Can u stop acting like u did bad. You know you did good but you post in here lookin for what? Praise? 165 means u think u got above 80 in the 2 papers. That’s more than enough for an a*
But as it used sigma notation, from 4 to 9, there are only 5 rectangles, with heights sqrt(4), sqrt(5), sqrt(6), sqrt(7), sqrt(8), sqrt(9) respectively. Each of these has a base length of dx so the total area, which is the answer we want, is the sum of these squares multiplied by dx. As a sum of these 5 sqrt terms is a normal number, we can write it as "k". We are at this point left with kdx with dx->0. This is a simple limit and is just zero. Using an integral, although it may be the wanted method, if not actually a legitimate way of working here