Draw a right-angle triangle if you don't already have one to work with. A right-angle triangle is any triangle with a 90 degree interior angle for one of its corners.
Measure one of the sides that are joined to the 90 degree angle.
Square the number that you found out; this means you need to multiply the number by itself. Let's say the measurement was 3 inches. Multiply 3 by 3 and you get 9, so the square number is nine.
Measure the second side linked to the 90 degree angle.
Calculate the square number for this measurement. Assuming for this example that the number is 4, multiply 4 by 4 to get 16. So the square number of side A is 9 and the square number for side B is 16.
Don't measure the final side; the Pythagorean Theorem's states that this is pointless as you already have all the information you need to work this out. Add the square numbers for side A and B together. In this case, 9 add 16 equals 25.
Work out the square root of 25, which means to find out which number, when multiplied by itself, will equal 25. If you multiply 5 by 5 you come out with 25, so the answer is 5. Without measuring the third side of the right-angle triangle you now know that it is 5 inches long.
Measure the one of the two shorter sides of the triangle, unless you already know how long it is.
Square this number. Assuming it is 4 inches long, the square number will be 16.
Measure the hypotenuse. Let's say it is 5 inches long.
Find the square number of this measurement to get 25.
Minus the square number of the short side, 16, from the square number of the hypotenuse, 25. The number you get is the square number of the other short side, which in this case is 9.
Work out the square root of 9. The answer is 3, since 3 times 3 equals 9. So the three sides of the triangle measure, 3, 4, and 5 inches.