ADB is a right angle is because central angle ACB is 180° and ADB is an inscribed angle whose endpoints are the same as ACB.
ADB = 1/2(ACB)
ADB = 1/2(180) = 90
For the above to hold true: (1) C must be the center of the circle (2) AB must be a diameter of the center
Find two numbers whose sum is b (or 2 in this example) and whose product is c (or -8 in this example).
Two such numbers are -2 and + 4, which add to +2 and multiply to -8.
a2 - 2ab + b2