What do you specifically do not understand?
http://www.boardofstudies.nsw.edu.au...hs_ext1_05.pdf
with explanation and reasons please question 3d
When the two chords Ab and CD intersect each other at E, they satisfy the following condition.
.............(1)
AE = 4, AB = 7. So EB = ...?
CD = l, ED = x, so CE = ...?
Substitute these values in equation (1) and simplify.
The equation obtained is
Or
To get the minimum length of the chord CD, find dl/dx and equate it to zero. Then solve for x. Substitute this value in eq.(1) to find CF, then find CD.