
The sine rule, or law of sines, is a trigonometric equation that can be used to find the unknown side of a triangle when two angles and one side are given. The rule can be used for any triangle, not just right-angled triangles, and is particularly useful for oblique triangles. The rule is derived from the work of 7th-century Indian mathematician Brahmagupta, and states that the ratio of the side length of a triangle to the sine of the opposite angle is the same for all three sides.
| Characteristics | Values |
|---|---|
| Can sine laws be used on non-right triangles? | Yes, the sine law can be used to solve oblique triangles, which are non-right triangles. |
| What is the sine law? | The sine law, also known as the law of sines, is a mathematical relationship that relates the lengths of the sides of a triangle to the sines of its angles. |
| How does it work? | The law states that the ratio of the length of a side of a triangle to the sine of the angle opposite that side is the same for all three sides of the triangle. |
| What is the mathematical expression? | \(\frac{a}{\sin \alpha}=\frac{b}{\sin \beta}=\frac{c}{\sin \gamma}\) or \(\frac{\sin \alpha}{a}=\frac{\sin \beta}{b}=\frac{\sin \gamma}{c}\) |
| What is it used for? | The sine law is used to find the unknown angle or unknown side of a triangle. |
| What are the criteria? | ASA (Angle-Side-Angle) or AAS (Angle-Angle-Side) criteria. |
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What You'll Learn

The Sine Law can be used to solve oblique triangles
The Sine Law, also known as the Law of Sines, can be used to solve oblique triangles. An oblique triangle is any triangle that is not a right triangle and could be an acute triangle (all angles less than 90 degrees) or an obtuse triangle (one angle greater than 90 degrees).
The Sine Law is based on proportions and can be presented in two ways. Firstly, as:
> sin(α)/a = sin(β)/b = sin(γ)/c
Secondly, as:
> a/sin(α) = b/sin(β) = c/sin(γ)
Where a, b, and c are the lengths of the sides of a triangle, and α, β, and γ are the angles opposite those sides.
The Sine Law can be used to find unknown angles or sides of a triangle when certain combinations of measurements are given. This is known as triangulation. For example, if we know two angles and one side, or two angles and the included side, we can use the Sine Law to find the unknown side.
The Sine Law is typically used to solve for unknowns in non-right triangles, where angle measures are not known. In a right triangle, the Pythagorean theorem is generally preferred as it is simpler and more direct. However, the Sine Law can still be applied to right triangles if needed.
The Sine Law has its roots in the astronomical and trigonometric work of 7th-century Indian mathematician Brahmagupta.
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The Sine Law is used to find unknown angles or sides
The Sine Law, also known as the Sine Rule or the Sine Formula, is a trigonometric equation used to find the unknown sides or angles of a triangle. It is based on the principle that the ratio of the sides of a triangle is equal to the ratio of the sines of the angles opposite those sides. In other words, for sides a, b, and c, and angles α, β, and γ, the equation can be written as:
> {displaystyle {frac {a}{sin {alpha }}}\,=\,{frac {b}{sin {beta }}}\,=\,{frac {c}{sin {gamma }}}}
The law of sines can be used to find the unknown sides or angles of a triangle when either two angles and one side are known, or two sides and one non-included angle are given. This technique is known as triangulation.
The Sine Law is particularly useful for solving oblique triangles, which are non-right triangles. An oblique triangle can be an acute triangle, where all three angles are less than right angles, or an obtuse triangle, where one of the angles is greater than a right angle.
To solve an oblique triangle, one can use any pair of applicable ratios. For instance, with the side of length a as the base, the triangle's altitude can be computed as b sin γ or as c sin β. Equating these two expressions gives:
> {displaystyle {frac {b}{sin {beta }}}\ =\ {frac {c}{sin {gamma }}}}
The Sine Law can also be used in real-life applications such as engineering, astronomy, and navigation.
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Sine Law is typically used to solve for unknowns in non-right triangles
The sine law, or the law of sines, is a trigonometric equation relating the lengths of a triangle's sides to the sines of its angles. The law of sines can be used to determine the unknown sides of a triangle when two angles and one side are known, or when two sides and the angle opposite them are known. In the former case, the sine law can be used to compute the remaining sides of a triangle, a technique known as triangulation.
The law of sines is based on proportions and can be presented in two ways. The first is:
> {\displaystyle {\frac {\sin {\alpha }}{a}}\,=\,{\frac {\sin {\beta }}{b}}\,=\,{\frac {\sin {\gamma }}{c}}
The second is:
> {\displaystyle {\frac {a}{\sin {\alpha }}}\,=\,{\frac {b}{\sin {\beta }}}\,=\,{\frac {c}{\sin {\gamma }}}
Here, a, b, and c are the lengths of the sides of a triangle, and α, β, and γ are the opposite angles.
The sine law can be used to solve oblique triangles, which are non-right triangles. In any triangle, an altitude, or a perpendicular line, can be drawn from one vertex to the opposite side, forming two right triangles. However, it is preferable to have methods that can be applied directly to non-right triangles without first dividing them into right triangles. The sine law can be used to solve for unknown sides and angles in these non-right triangles.
The sine law can be used in any triangle, not just right-angled triangles, where a side and its opposite angle are known. This is known as the sine rule.
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Sine Law can be used to compute the remaining sides of a triangle
The sine law, also known as the law of sines, is a mathematical relationship that relates the lengths of the sides of a triangle to the sines of its angles. The law states that the ratio of the length of a side of a triangle to the sine of the angle opposite that side remains constant for all three sides of the triangle. The sine law can be used to compute the remaining sides of a triangle when two angles and one side are known, or when two sides and one angle are known. This technique is known as triangulation.
The sine law is generally used to find the unknown angle or side of a triangle when certain combinations of measurements of a triangle are given. There are two criteria that will provide a unique solution: the ASA (Angle-Side-Angle) criterion and the AAS (Angle-Angle-Side) criterion. The ASA criterion requires two angles and the included side to find the unknown side, while the AAS criterion requires two angles and a non-included side.
The sine law can be applied to both right triangles and non-right triangles. However, it is important to note that the sine law is typically used to solve for unknowns in non-right triangles, as in right triangles, the Pythagorean theorem is generally preferred due to its simplicity and directness. In a right triangle, one angle is already known (90 degrees), so there are only two angles to consider. While the sine law can be used to find unknown side lengths or angles in a right triangle, it is not the most efficient method, and the Pythagorean theorem is usually the preferred approach.
The sine law has its roots in the 7th century, with the work of Indian mathematician Brahmagupta. Brahmagupta expressed the circumradius of a triangle as the product of two sides divided by twice the altitude. The law of sines can be derived by expressing the altitude as the sine of one of the base angles times its opposite side and then equating the two resulting variants. The spherical law of sines is credited to 10th-century scholars Abu-Mahmud Khujandi and Abū al-Wafāʾ, and it was later given prominence by Abū Naṣr Manṣūr in his "Treatise on the Determination of Spherica."
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Sine Law is used in engineering to measure the angle of tilt
The sine law, also known as the sine rule or sine formula, is a trigonometric equation relating the lengths of the sides of any triangle to the sines of its angles. In other words, the ratio of the side length of a triangle to the sine of the opposite angle is the same for all three sides. The sine rule can be used to find the unknown length or angle of a triangle when two angles and one side are known, or when two sides and one non-included angle are given.
The sine rule is used in engineering to measure the angle of tilt. One example of this is the sine bar, a precision measuring instrument used to measure the angular deviation or tilt of a surface with reference to a level. The sine bar is made of high carbon, high chromium steel, which is corrosion-resistant. It consists of two precision-ground cylinders attached to each end of a hardened, precision-ground body. The distance between the cylinder centres is precisely controlled, and the bar's top is parallel to the centres of the two rollers. When in use, the dimension between the two rollers is chosen to be a whole number to facilitate subsequent calculations, and this forms the hypotenuse of a triangle. When placed on a flat surface, its top edge will be parallel to it.
The sine rule is also used in metalworking and construction. For example, when one roller is raised by a known amount, the top edge of the bar is tilted by the same amount, yielding an angle calculated with the sine rule. The sine rule can also be used to measure angles between structures such as stairs and walls.
The sine rule can be used to solve oblique triangles, which are non-right triangles. In any triangle, an altitude, or a perpendicular line from one vertex to the opposite side, can be drawn to form two right triangles. However, it is preferable to have methods that can be applied directly to non-right triangles without first having to create right triangles.
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