In a right-angled triangle the square of the hypotenuse is the sum of the squares of the
other two sides. More free lessons at: http://www.khanacademy.org/video?v=eF6zYNzlZKQ Note: All Khan Academy content is available for free at (www.khanacademy.org)

Each exterior angle of a triangle is equal to the sum of the interior opposite angles. More free lessons at: http://www.khanacademy.org/video?v=eF6zYNzlZKQ Note: All Khan Academy content is available for free at (www.khanacademy.org)

Proof that the sum of the measures of the angles in a triangle are 180 degrees. More free lessons at: http://www.khanacademy.org/video?v=eF6zYNzlZKQ Note: All Khan Academy content is available for free at (www.khanacademy.org)

How to prove theorem 12: Let ABC be a triangle. If a line l is parallel to BC and cuts
[AB] in the ratio s:t, then it also cuts [AC] in the same ratio (and converse).