# When Would You Use Law of Sines

Align the proportion with 2 pairs of opposite sides/sine of angles A purely algebraic proof can be constructed from the spherical law of cosine. From the sin identity 2 A = 1 − cos 2 A {displaystyle sin ^{2}A=1-cos ^{2}A} and the explicit expression for cos A {displaystyle cos A} of the spherical law of cosine The sine law and the cosine law are used to find the unknown angle or side of a triangle. Let us contrast the difference between the two laws. For a general triangle, the following conditions should be met for the case to be ambiguous: By substituting K = 0, K = 1 and K = −1, we obtain the Euclidean, spherical and hyperbolic cases of the sinus law described above. The law of sine is usually used to find the angle or unknown side of a triangle. This law can be used when certain combinations of measurements of a triangle are given. In general, the law of the sine is defined as the ratio of the length of the side to the sine of the opposite angle. It applies to all three sides of a triangle or their sides and angles. Let pK(r) be the circumference of a circle of radius r in a space of constant curvature K. Then pK(r) = 2π sinK r.

Therefore, the law of sins can also be expressed as: The image below shows a case that does not fit the law of sines. Since we do not know of opposite sides and angles, we cannot apply the formula. Can the law of sine be used to solve the marked angle? Now use the formula of the sinusoidal law to make an equation. When the sine law is used to find one side of a triangle, an ambiguous case occurs when two separate triangles can be constructed from the data provided (i.e. there are two different possible solutions to the triangle). In the case shown below, these are the triangles ABC and ABC`. The law of sine in constant curvature K reads as follows These are opposites: To use the law of sine, you need to know a measurement of opposite angle/side pair. The law of sine is used to determine the unknown side of a triangle when two angles and sides are given.

The sinusoidal law defines the ratio of the sides of a triangle and their respective sinusoidal angles are equivalent to each other. Other names for the sinusoidal distribution are sinusoidal distribution, sinusoidal rule, and sinusoidal formula. So we use the sinusoidal rule to find unknown lengths or angles of the triangle. It is also known as the sinusoidal rule, sinusoidal law or sinusoidal formula. When looking for the unknown angle of a triangle, the formula of the sinusoidal law can be written as follows: Or just look at it: we can use the formula if we have 2 sides and the angle not closed. It is easy to see how for small spherical triangles, if the radius of the sphere is much larger than the sides of the triangle, this formula becomes the plane formula at the limit because the formula of the sinusoidal law allows us to set up a part of the opposite side/angle (ok, in fact you take the sine of an angle and its opposite side). (Question from our law of sins downloadable pdf worksheeet) In hyperbolic geometry, if the curvature is −1, the law of sine considers a triangle where we obtain a, b and A. (The height h from vertex B to side A C ̄ is equal to b sin A according to the definition of the sine.) In trigonometry, the sinusoidal law or sinusoidal law is an equation that relates the lengths of the sides of a triangle to the sine of its angles. According to the law, the law of sine on surfaces with constant curvature can be generalized to higher dimensions.

 If we get two sides and a closed angle of a triangle, or if we get 3 sides of a triangle, we cannot apply the law of sines because we cannot establish proportions for which enough information is known. In both cases, we must apply the law of cosine. Ibn Muʿādh al-Jayyānīs The Book of Unknown Arcs of a Sphere in the 11th Century contains the general law of the sine.  The Sines law of levels was introduced later in the 13th century. It was erected by Nasīr al-Dīn al-Tūsī in the nineteenth century. In his book On the Sector Figure he presented the law of sine for plane and spherical triangles and provided evidence for this law.  The trick to knowing when to apply Sines` law is to draw an image and determine which parts of the triangle are known and which parts are missing. In general, the law of sine is used to solve the triangle when we know two angles and one side or two angles and one side closed.

This means that the sine law can be used if we have ASA (angle-side-angle) or AAS (angle-angle-angle-side) criteria. Note that the possible solution α = 147.61° is excluded, as this would necessarily result α + β + γ > 180°. According to Ubiratàn D`Ambrosio and Helaine Selin, the spherical law of sins was discovered in the 10th century. It is attributed to various Abu-Mahmud Khojandi, Abu al-Wafa` Buzjani, Nasir al-Din al-Tusi and Abu Nasr Mansur.  The answers are almost the same! (They would be exactly the same if we used perfect precision). This means that if we divide side a by the sine of ∠A, it is equal to dividing side b by the sine of ∠ B, and also equal to dividing side c by sine of ∠C (or) The sides of a triangle are in the same proportion to each other as the sins of their opposite angles.