What is the missing constant term in the perfect square that starts with x^2-20xx 2 −20xx, squared, minus, 20, x ?

Answer 1

When you have

x² + bx,

you can complete the square by adding b²/4.

b = -20

b²/4 = (-20)²/4 = 100

x² + bx + b²/4 = x² - 20x + 100 = (x - 10)²

You can see it is a perfect square.