Using section formula

Example
ABCD is a rectangle whose three vertices are B (4, 0), C(4, 3) and D(0, 3). Calculate the length of one of its diagonals.
Solution:

AB = 4 units
BC = 3 units

A{C^2} = A{B^2} + B{C^2}     (Pythagoras theorem)

\begin{array}{l} = {(4)^2} + {(3)^2} \\ = 16 + 9 = 25\\ \therefore AC = 5cm \\ \end{array}

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