oblique triangle
C1Academic, Technical
Definition
Meaning
A triangle that does not contain a right angle (90-degree angle).
In geometry, any triangle that is not a right triangle, meaning all three angles are either acute (less than 90 degrees) or one angle is obtuse (greater than 90 degrees). The term is used in trigonometry to contrast with right triangles, for which simpler formulas apply.
Linguistics
Semantic Notes
The term is exclusively used in geometry and trigonometry. It is a categorical term defining what a triangle is *not* (i.e., not right-angled). In common parlance, people simply say "triangle"; specifying "oblique" is only necessary in mathematical contexts to highlight the absence of a right angle.
Dialectal Variation
British vs American Usage
Differences
No significant differences in meaning or usage. Spelling of related terms follows regional conventions (e.g., 'centre' vs. 'center').
Connotations
Purely technical, no connotative differences.
Frequency
Equally low frequency in both dialects, confined to mathematical education and practice.
Vocabulary
Collocations
Grammar
Valency Patterns
[The/An] oblique triangle [has/contains] [three angles].To solve [for a side/an angle] in [an/the] oblique triangle, use [the Law of Sines/Cosines].Vocabulary
Synonyms
Neutral
Weak
Vocabulary
Antonyms
Usage
Context Usage
Business
Virtually never used.
Academic
Used in mathematics, engineering, and physics courses, specifically in trigonometry and geometry modules.
Everyday
Extremely rare outside of educational help or specific technical discussions.
Technical
Core term in trigonometry, surveying, computer graphics, and any field requiring triangular calculations without right angles.
Examples
By Part of Speech
adjective
British English
- The problem involved an oblique triangle.
- We need oblique triangle formulae.
American English
- The diagram shows an oblique triangle.
- Apply the oblique triangle solution method.
Examples
By CEFR Level
- A triangle with all acute angles is an oblique triangle.
- A right triangle has a 90-degree angle, but an oblique triangle does not.
- To find the missing side of the oblique triangle, you cannot use the Pythagorean theorem.
- Surveyors often deal with oblique triangles when measuring land that isn't perfectly rectangular.
- The maritime navigation problem was reduced to solving a spherical oblique triangle using haversine formulas.
- In computational geometry, decomposing a polygon into oblique triangles can simplify certain algorithms.
Learning
Memory Aids
Mnemonic
Think 'OBLIQUE = Off-Beat, Lacking In Right-Angle Queen's Edict.' It's a triangle that doesn't obey the right-angle rule.
Conceptual Metaphor
A DEVIATION FROM THE STANDARD. The right triangle is often the default or simplest case ('straight' morally and mathematically); an oblique triangle represents a more complex, non-standard situation requiring advanced tools (like the Law of Sines).
Watch out
Common Pitfalls
Translation Traps (for Russian speakers)
- Avoid direct translation of 'oblique' (косой) implying 'slanted' in a physical sense. The term is a fixed mathematical category.
- Do not confuse with 'arbitrary triangle' (произвольный треугольник). 'Oblique' specifically means non-right-angled.
Common Mistakes
- Mispronouncing 'oblique' as /oʊˈblaɪk/ or /ˈɒblɪk/.
- Using 'oblique triangle' to describe a triangle that is physically slanted on a page (orientation is irrelevant).
- Confusing it with an 'isosceles' or 'scalene' triangle (which are subcategories; an oblique triangle can be either).
Practice
Quiz
Which of the following best describes an oblique triangle?
FAQ
Frequently Asked Questions
Yes. 'Oblique triangle' is precisely the term for any triangle without a 90-degree angle.
No, by definition. If it has a 90-degree angle, it is a right triangle.
Acute triangles (all angles < 90°) and obtuse triangles (one angle > 90°).
Primarily in trigonometry courses when learning the Law of Sines and Law of Cosines, which are required for solving triangles without right angles.