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The solution by @MathLover1 is INCORRECT.
I came to bring the correct solution.
Let the given triangle be ABC, with the right angle B;
the leg AB = 12;
leg BC = 9,
and the hypotenuse AC = 15.
The bisector AD of the acute angle A divides the leg BC in two parts BD and DC.
Then the ratio |BD| to |CD| is equal to the ratio |AB| to |AC|, i.e. = .
It implies that |BD| = 4, |BC| = 5.
Hence, the length of the bisector AD is equal to = = . ANSWER
Solved (C O R R E C T L Y).
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Looking into her activity in last several days, I start thinking that
@MathLover1 presents a real danger for any visitor to this forum, because she provides wrong solutions even to simplest Math problems.
See my notes to her posts in past two days
https://www.algebra.com/algebra/homework/Finance/Finance.faq.question.1177850.html
https://www.algebra.com/algebra/homework/Volume/Volume.faq.question.1177847.html
https://www.algebra.com/algebra/homework/Surface-area/Surface-area.faq.question.1177846.html
https://www.algebra.com/algebra/homework/Triangles/Triangles.faq.question.1177800.html
https://www.algebra.com/algebra/homework/Triangles/Triangles.faq.question.1177804.html