let us use engineering notation, for the sake of shortening the zeros expression, so 100,000 is just 1e5, namely 1 followed by 5 zeros, and 399 million is just 399e6, or 399 followed by 6 zeros and so on.
check the picture below.
cd is a diameter, that means the arccabd = 180°.
since cd and ab are both parallels, the arcs they make are congruents, namely arcca = arcbd.
let's say arcca is hmmmm "x".