Calculating whether right triangles are similar - GMAT Quantitative

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Question

Which set of side lengths cannot be the side lengths of a right triangle?

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Answer

For a triangle to be a right triangle, the sides must obey the Pythagorean Theorem. Let's try our options.

3, 4, 5: You should know this is a right triangle without having to do any calculations because it is one of the special triangles that you should remember. But if you didn't, $3^{2}$ + $4^{2}$ = 25 = $5^{2}$.

28, 45, 53: $28^{2}$ + $45^{2}$ = 784 + 2025 = 2809 = $53^{2}$

45, 55, 75: $45^{2}$ + $55^{2}$ = 2025 + 3025 = 5050 neq $75^{2}$. The sides don't follow the Pythagorean Theorem so this can't be a right triangle. This is our answer. Let's check the remaining two sets of sides as well.

48, 64, 80: $48^{2}$ + $64^{2}$ = 2304 + 4096 = 6400 = $80^{2}$. These are pretty big numbers and this math might take a while. Instead of doing these calculations, we could also see if 48, 64, and 80 look like any of the special triangles we know. Let's divide the three numbers by 16. 48/16 = 3, 64/16 = 4, and 80/16 = 5. Then this is just a type of 3,4,5 triangle, which we know is a right triangle.

84, 35, 91: $84^{2}$ + $35^{2}$ = 7056 + 1225 = 8281 = $91^{2}$. Again, these are big numbers to square. Let's divide the three numbers by their greatest common factor, 7. 84/7 = 12, 35/7 = 5, 91/7 = 13. Then this is a 5, 12, 13 triangle, which is another of our special triangles that we know is a right triangle.

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