To factorize the quadratic expression x^2-9x+20x2−9x+20, one must find the two numbers that add together to give 9, and multiply together to give 20.
Those two numbers are 4 and 5, which factor into the expression (x-4)(x-5)(x−4)(x−5). The signs are due to the fact that the first sign in the expression is a positive, which tells us that both signs must be the same, and the first sign is a negative, which means that, due to the positive sign in the expression, both signs are negative.
To do the opposite, expanding, one must multiply xx by xx, to give x^2x2, add together 44 and 55 to give 99, and multiply them to give 2020.
