I wanna know: [3,5]={3,4,5}? Why?

Yes if [3,5] is a set of integers then it contains elements 3,4,5

As Closed bracket denotes that extreme points are also included and 4 is mid element in between 3 and 5

step-by-step explanation:

given that ab is divided into equal parts as

ac, cd, de, ef, fg, gh, hi, ib.

from this we find that f is the midpoint of ab.

so using mid point formula we find

f = (3,7)

similarly considering fb, we get

h = mid point of fb = ((6,6) and g, mid point of fh is

g = (9/2, 13/2)

i is mid point of hb

so i =(15/2,11/2)