Oblique Angle — Definition, Formula & Examples
An oblique angle is any angle that is not exactly 0°, 90°, 180°, or 360°. In everyday use, it most often refers to an angle that is neither a right angle nor a straight angle.
An angle is oblique if and only if it is not a right angle (90°) and not a straight angle (180°). Both acute angles (between 0° and 90°) and obtuse angles (between 90° and 180°) are oblique angles.
How It Works
To determine whether an angle is oblique, check if it measures exactly 90° or 180°. If it does, it is not oblique. Any other positive angle measure (such as 30°, 45°, 120°, or 150°) qualifies as oblique. The term appears frequently when describing triangles: an oblique triangle is any triangle that does not contain a right angle.
Example
Problem: Classify each angle as oblique or not oblique: 45°, 90°, 135°, 180°.
Check 45°: 45° is not 90° and not 180°, so it is oblique (it is also acute).
Check 90°: 90° is a right angle, so it is not oblique.
Check 135°: 135° is not 90° and not 180°, so it is oblique (it is also obtuse).
Check 180°: 180° is a straight angle, so it is not oblique.
Answer: 45° and 135° are oblique angles. 90° and 180° are not.
Why It Matters
The term "oblique" shows up when you study oblique triangles in trigonometry, where you use the Law of Sines and the Law of Cosines instead of basic right-triangle ratios. Recognizing whether an angle or triangle is oblique helps you choose the correct solving method.
Common Mistakes
Mistake: Thinking oblique means the same as obtuse.
Correction: Oblique includes both acute and obtuse angles — any angle that is not a right angle or straight angle. A 40° angle is oblique just as much as a 140° angle is.