Article in English: A right triangle is a triangle that has one right (90 degree) angle.  The side of the triangle opposite the right angle is always the longest side, and it is called the hypotenuse.   Right triangles show up frequently on math tests, and fortunately there is a very handy formula for finding the length of unknown sides! The Pythagorean Theorem tells us that for any right triangle with sides of length a and b, and hypotenuse of length c, a2 + b2 = c2. Remember that the longest side of the triangle is called the hypotenuse.  It will be opposite the right angle and must be labeled c.  Label the two shorter sides a and b.  It doesn't really matter which is which, the math will turn out the same! Remember that a2 + b2 = c2.  Substitute the side lengths in for the corresponding letters in the equation.  If, for example, you know that side a = 3 and side b = 4, then plug those values into the formula as follows: 32 + 42 = c2. If you know the length of side a = 6, and the hypotenuse c = 10, then you should set the equation up like so: 62 + b2 = 102. You will first need to square the known side lengths which means multiplying each value by itself (for example 32 = 3 * 3 = 9).  If you are looking for the hypotenuse, simply add the two values together and find the square root of this number to find the length.  If it is a side length you are missing, you must do a bit of easy subtraction, and then take the square root to get your side length.  In the first example, square the values in 32 + 42 = c2 and find that 25= c2. Then calculate the square root of 25 to find that c = 5. In the second example, square the values in 62 + b2 = 102 to find that 36 + b2 = 100.  Subtract 36 from each side to find that b2 = 64, then take the square root of 64 to find that b = 8. Recall that the perimeter P = a + b + c.  Now that you know the lengths of sides a, b and c, you simply need to add the lengths together to find the perimeter.  In our first example,P = 3 + 4 + 5, or 12. In our second example, P = 6 + 8 + 10, or 24.

Summary in Vietnamese:
Nhớ lại thế nào là một tam giác vuông. Nhớ lại định lý Pytago. Nhìn vào tam giác của bạn và đặt tên các cạnh "a", "b" và "c". Nhập chiều dài cạnh mà bạn đã biết vào định lý Pytago. Giải phương trình, tìm độ dài cạnh còn thiếu. Cộng chiều dài ba cạnh của tam giác để tìm chu vi của nó.