Article in English: The Pythagorean Theorem describes the relationship between the sides of a right triangle.  It states that for any right triangle with sides of length a and b, and hypotenuse of length c, a2 + b2 = c2. The Pythagorean Theorem only works on right triangles, and by definition only right triangles can have a hypotenuse.  If your triangle contains one angle that is exactly 90 degrees, it is a right triangle and you can proceed. Right angles are often notated in textbooks and on tests with a small square in the corner of the angle.  This special mark means "90 degrees." The variable "c" will always be assigned to the hypotenuse, or longest side.  Choose one of the other sides to be a, and call the other side b (it doesn't matter which is which; the math will turn out the same).  Then copy the lengths of a and b into the formula, according to the following example: If your triangle has sides of 3 and 4, and you have assigned letters to those sides such that a = 3 and b = 4, then you should write your equation out as: 32 + 42 = c2. To find the square of a number, you simply multiply the number by itself, so a2 = a x a.  Find the squares of both a and b, and write them into your formula.  If a = 3, a2 = 3 x 3, or 9.  If b = 4, then b2 = 4 x 4, or 16. When you plug those values into your equation, it should now look like this: 9 + 16 = c2. Enter this into your equation, and this will give you the value for c2. There is only one step left to go, and you will have that hypotenuse solved! In our example, 9 + 16 = 25, so you should write down 25 = c2. Use the square root function on your calculator (or your memory of the multiplication table) to find the square root of c2.   The answer is the length of your hypotenuse! In our example,  c2 = 25.  The square root of 25 is 5 (5 x 5 = 25, so Sqrt(25) = 5).  That means c = 5, the length of our hypotenuse!

Summary in Vietnamese:
Học định lý Pytago. Đảm bảo rằng tam giác của bạn là tam giác vuông. Gán biến a, b và c vào các cạnh trong tam giác của bạn. Tìm bình phương của a và b. Cộng các giá trị a2 và b2 với nhau. Tìm căn bậc hai của c2.