Angles around a line add up to 180°. Angles around a point add up to 360°. When two lines intersect, adjacent angles are supplementary (they add up to 180°) and angles across from either other are vertical (they're equal).

A trapezoid is a quadrilateral with one set of parallel sides.

The first step to solve a quadratic equation that's set to zero is to factor the quadratic equation:

x² - 8x - 9 = 0
(x + 1)(x - 9) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (x + 1) or (x - 9) must equal zero:

If (x + 1) = 0, x must equal -1
If (x - 9) = 0, x must equal 9

So the solution is that x = -1 or 9

To combine like terms, add or subtract the coefficients (the numbers that come before the variables) of terms that have the same variable raised to the same exponent.

6a⁸ - 9a⁸ = -3a⁸

A quadrilateral is a shape with four sides. The perimeter of a quadrilateral is the sum of the lengths of its four sides.