According to the Pythagorean theorem AB=5 and the similarity of triangles, we can get AC: AB = a1c: AC = 4/5a1c = 4/5ac. a 1c:AC = a 1c 1:a 1c = 4/5 a 1c 1=4/5a 1c=(4/5)^ 2ac a 1c 1:a 1c = a2c 1:a 1c 1 = 4/5

A2c1= 4/5a1ca1= (4/5) 2a1c = (4/5) 3ac, so we can get A5C5 = (4/5)10ac = 3.