The length is 11.5 cm.
The sum of the squares on the legs of a right triangle is equal to the square on the hypotenuse
Given: diagonal = 20 cm
In ABC,
AC² = BC² + AB²
AC²= x² + x²
AC²= 2x²
AC= √2 x
IN ACF,
AF² = AC² +CF²
20²= 2x² + x²
400= 3x²
400/3= x²
x= 11.5
hence, length is 11.5 cm.
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