Shortcut Trick
For a right-angled triangle, the lengths of the sides must form a valid Pythagorean triplet.
Standard Pythagorean triplets frequently asked in exams include (3, 4, 5), (5, 12, 13), (8, 15, 17), and (7, 24, 25).
By direct observation, the set {5, 12, 13} exactly matches a standard Pythagorean triplet.
∴ The correct answer is {5, 12, 13}.
Alternate Method
Given:
Options with sets of side lengths: {3, 4, 6}, {8, 15, 16}, {5, 10, 12}, and {5, 12, 13}
Formula Used:
According to the Pythagoras Theorem, for a triangle to be right-angled, the sum of the squares of the two smaller sides must be equal to the square of the longest side (hypotenuse):
a2 + b2 = c2
Base = 12 Height = 5 Hypotenuse = 13
Calculations:
⇒ Let's check Option 1: 32 + 42 = 9 + 16 = 25. The square of the third side is 62 = 36. Since 25 ≠ 36, it is not a right-angled triangle.
⇒ Let's check Option 2: 82 + 152 = 64 + 225 = 289. The square of the third side is 162 = 256. Since 289 ≠ 256, it is not a right-angled triangle.
⇒ Let's check Option 3: 52 + 102 = 25 + 100 = 125. The square of the third side is 122 = 144. Since 125 ≠ 144, it is not a right-angled triangle.
⇒ Let's check Option 4: 52 + 122 = 25 + 144 = 169. The square of the third side is 132 = 169. Since 169 = 169, it perfectly satisfies the theorem.
∴ The correct answer is {5, 12, 13}.
Additional Information
Pythagoras Theorem
In a right-angled triangle, the square of the hypotenuse is exactly equal to the sum of the squares of the other two sides: Hypotenuse2 = Base2 + Perpendicular2.
Common Pythagorean Triplets
Some basic integer triplets to memorize are (3, 4, 5), (5, 12, 13), (8, 15, 17), (7, 24, 25), and (9, 40, 41). Any constant multiple of these triplets (e.g., 6, 8, 10) also forms a right-angled triangle.
Identifying Triangle Types
If c is the longest side: If a2 + b2 > c2, the triangle is an acute-angled triangle. If a2 + b2 < c2, the triangle is an obtuse-angled triangle.