# Quadrilaterals

8 videos

1 skill

Not all things with four sides have to be squares or rectangles! We will now broaden our understanding of quadrilaterals.

### Proof: Opposite sides of a parallelogram are congruent (and conversely)

VIDEO
8:30 minutes

Sal proves that a figure is a parallelogram if and only if opposite sides are congruent.

### Proof: The diagonals of a parallelogram bisect each other (and conversely)

VIDEO
9:06 minutes

Sal proves that a quadrilateral is a parallelogram if and only if its diagonals bisect each other.

### Proof: Opposite angles of parallelogram are congruent

VIDEO
4:08 minutes

Sal proves that opposite angles of a parallelogram are congruent.

### Quadrilateral angles

PRACTICE PROBLEMS

### Proof: Rhombus diagonals are perpendicular bisectors of each other

VIDEO
4:04 minutes

Sal proves that the diagonals of a rhombus are perpendicular, and that they intersect at the midpoints of both.

### Proof: Rhombus area is half the product of the lengths of its diagonals

VIDEO
5:07 minutes

Sal proves that we can find the area of a rhombus by taking half the product of the lengths of the diagonals.

### Area of a parallelogram proof

VIDEO
5:42 minutes

Guess what's interesting about the opposite sides of a parallelogram? That's right....they are parallel! Let's find the area--base times height

### Whether a special quadrilateral can exist

VIDEO
5:08 minutes

Proving whether a special quadrilateral can exist

### Rhombus diagonals

VIDEO
4:38 minutes

Proof that the diagonals of a rhombus are perpendicular bisectors of each other