Question

What number must be added to the expression below to complete the
square?
x² - 18x​

Answer 1

The number that has to be added is the one that isn’t added

Answer 2

+81

Explanation:

remember the formula (a-b)^2=a^2+b^2-2ab

x^2=a^2

-18x=-2*a*b

and you're trying to find b^2

you can get b from the 2*a*b

if -2*a*b=-18x, well you can remove a and x first, so -2b=-18 and b=9

9^2 is 81