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.

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