Answer:
AD/AB = AB/AC
Step-by-step explanation:
If we take two triangules in that figure: ABC and ABD, both are triangles rectangles. We need to use the definition of cosine, with the angle in th A corner, we called ∅.
For the ABC triangle: cos(∅) = adjacent cathetus/hypotenuse = AD/AB
For the ABD triangle: cos(∅) = adjacent cathetus/hypotenuse = AB/AC
Then:
cos(∅) = AD/AB = AB/AC
