Shortcut Trick
For any triangle with sides a, b, and c (where c is the longest side), the type of triangle is determined by comparing a2 + b2 and c2.
Here, the sides are proportional to 7, 24, and 30. Let a = 7, b = 24, and c = 30.
Calculate: 72 + 242 = 49 + 576 = 625.
Calculate: 302 = 900.
Since 900 > 625 (i.e., c2 > a2 + b2), the triangle is obtuse angled.
∴ The correct answer is obtuse angled.
Alternate Method
Given: The ratios of the lengths of the sides of a triangle are 7 : 24 : 30.
Formula Used:
- Triangle Inequality: Sum of any two sides must be greater than the third side (a + c).
- Cosine Property:
• If a2 + b2 > c2 → Acute-angled triangle.
• If a2 + b2 = c2 → Right-angled triangle.
• If a2 + b2 < c2 → Obtuse-angled triangle.
Calculations:
⇒ Let the sides be a = 7k, b = 24k, and c = 30k.
⇒ First, check triangle validity: 7k + 24k = 31k. Since 31k > 30k, the triangle is possible.
⇒ Now, compare the squares of the sides: a2 = (7k)2 = 49k2.
⇒ b2 = (24k)2 = 576k2.
⇒ c2 = (30k)2 = 900k2.
⇒ Sum of squares of smaller sides = 49k2 + 576k2 = 625k2.
⇒ Comparing a2 + b2 and c2: 625k2 < 900k2.
⇒ Since a2 + b2 < c2, the angle opposite to side c is obtuse (greater than 90°).
∴ The correct answer is obtuse angled.
Additional Information
Triangle Inequality Theorem
The sum of the lengths of any two sides of a triangle must be strictly greater than the length of the third side (a + c).
Pythagorean Triplets
If a2 + b2 = c2, the triangle is right-angled. Common examples include (3, 4, 5), (5, 12, 13), and (7, 24, 25).
Cosine Rule and Angles
For triangle ABC, cos C = (a2 + b2 − c2) ÷ (2ab). If the numerator is negative (a2 + b2 < c2), angle C is obtuse.