In the triangle ABC, the angle c = 90 °, ab = 2.1cm, and the angle BC = 2.8c, why is the height CD calculated by multiplying the area by 2 and then dividing the hypotenuse

In the triangle ABC, the angle c = 90 °, ab = 2.1cm, and the angle BC = 2.8c, why is the height CD calculated by multiplying the area by 2 and then dividing the hypotenuse

Triangle area = bottom * height / 2
Because the angle c = 90 degrees, there are two ways to find the area of this triangle
Area = AC * BC / 2 = AB * CD / 2
So CD = area * 2 / AB

In the triangle ABC, the angle ACB = 90 degrees, AC = 1.8cm, BC = 2.4cm, find the area of "1" triangle ABC s "2" hypotenuse AB "3" height CD
Solutions to Pythagorean theorem

S△ABC=AC×BC×0.5=2.16cm²
(2) Solution; ab & sup2; = AC & sup2; + AB & sup2; = 1.8 & sup2; + 2.4 & sup2; = 9
AB=3
AB×CD=AC×BC
3×CD=1.8×2.4
CD=1.44