The length of DF is 36.
What is tangent?
The tangent is defined as a line touching circles or an ellipse at only one point.
As, the tangent is perpendicular to the radius of the circle.
So, <CDF =90
Now, using Pythagoras theorem in CDF
CF² = FD² + DC²
39² = DF² + 15²
1521-225= DF²
DF = √1296
DF= 36.
Hence, the length of DF is 36.
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