The AD's length of 3m will enable the angle at B to be divided in half.
Angle Bisector Theorem: What is it?
The angle bisector of a triangle divides the opposing side into two portions that are proportional to the other two sides, according to the angle bisector theorem, in simpler words the angle bisector theorem is concerned with the relative lengths of the two segments that a triangle's side is divided into by a line that bisects the opposite angle. It equates their relative lengths to the relative lengths of the other two sides of the triangle.
The triangle has sides of 5, 7, and (x+4) m.
Angle B's angle bisector will only be the BD if
x/4 = 5/7
x = 5 *4 / 7
x = 20/7 = 2.85 ≈ 3m
Thus if AD has length of 3m then it will enable the angle at B to be divided in half.
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