Primitive Pythagorean triplet properties

Let’s discuss a few useful properties of primitive Pythagorean triples. A primitive Pythagorean triple is one in which a, b and c (the length of the two legs and the hypotenuse, respectively) are co-prime. So, for example, (3, 4, 5) is a primitive Pythagorean triple while its multiple, (6, 8, 10), is not. Now, without further ado, here are the properties of primitive Pythagorean triples that you’ll probably encounter on the CAT: I. One of a and b is odd and the other is even. II. From property I, we can then say that c is odd. III. Exactly one of a, b is divisible by 3. IV. Exactly one of a, b is divisible by 4. V. Exactly one of a, b, c is divisible by 5. If you keep in mind the first primitive Pythagorean triple that we used as an example (3, 4, 5), it is very easy to remember all these properties. If we look at some other examples: (3, 4, 5), (5, 12, 13), (8, 15, 17) (7, 24, 25) (20, 21, 29) (12, 35, 37) (9, 40, 41) (28, 45, 53) (11, 60, 61) (16, 63, 65) (33, 56, 65) (48, 55, 73), etc. we will see that these properties hold for all primitive Pythagorean triples. Now, let’s take a look at an example  question which can be easily solved if we know these properties: The three sides of a triangle have lengths p, q and r, each an integer. Is this triangle a right triangle? Statement 1: The perimeter of the triangle is an odd integer. Statement 2: If the triangle’s area is doubled, the result is not an integer. We know that the three sides of the triangle are all integers. So if the triangle is a right triangle, the three sides will represent a Pythagorean triple. Given that p, q and r are all integers, let’s use the properties of primitive Pythagorean triples to break down each of the statements. Statement 1: The perimeter of the triangle is an odd integer. Looking at the properties above, we know that a primitive Pythagorean triple can be represented as: (Odd, Even, Odd) (The first two are interchangeable.) Non-primitive triples are made by multiplying each member of the primitive triple by an integer n greater than 1. Depending on whether n is odd or even, the three sides can be represented as: (Odd*Odd, Even*Odd, Odd*Odd) = (Odd, Even, Odd) or (Odd*Even, Even*Even, Odd*Even) = (Even, Even, Even) However, the perimeter of a right triangle can never be odd because: Odd + Even + Odd = Even Even + Even + Even = Even Hence, the perimeter will be even in all cases. (If the perimeter of the given triangle is odd, we can say for sure that it is not a right triangle.) This statement alone is sufficient. Statement 2: If the triangle’s area is doubled, the result is not an integer. If p, q and r are the sides of a right triangle such that r is the hypotenuse (the hypotenuse could actually be either p, q, or r but for the sake of this example, let’s say it’s r), we can say that: The area of this triangle = (1/2)*p*q and Double of area of this triangle = p*q Double the area of the triangle has to be an integer because we are given that both p and q are integers, but this statement tells us that this is not an integer. In that case, this triangle cannot be a right triangle. If the triangle is not a right triangle, double the area would be the base * the altitude, and the altitude would not be an integer in this case. This statement alone is sufficient, too. Therefore, our answer is D. As you can see, understanding the special properties of primitive Pythagorean triples can come in handy on the CAT – especially in tackling complicated geometry questions.