# How to know when to use law of sines or cosines

## How do you know when to use law of cosines?

When to Use

The Law of Cosines is useful for finding: the third side of a triangle when we know two sides and the angle between them (like the example above) the angles of a triangle when we know all three sides (as in the following example)

## How can you tell when to use Sin Cos or tan?

If you have the hypotenuse and the opposite side, then use sine. If you have the hypotenuse and the adjacent side, then use cosine. If you have the adjacent and the opposite sides, then use tangent.

## What is the difference between sine rule and cosine rule?

The sine rule can be used to find an angle from 3 sides and an angle, or a side from 3 angles and a side. The cosine rule can find a side from 2 sides and the included angle, or an angle from 3 sides.

## Can you use the law of sines on a right triangle?

The Law of Sines says that in any given triangle, the ratio of any side length to the sine of its opposite angle is the same for all three sides of the triangle. This is true for any triangle, not just right triangles. Press ‘reset’ in the diagram above.

## Does law of cosines work for all triangles?

It works on any triangle, not just right triangles. where a and b are the two given sides, C is their included angle, and c is the unknown third side. See figure above.

## Which Triangle is a 30 60 90 Triangle?

A 30-60-90 triangle is a special right triangle whose angles are 30º, 60º, and 90º. The triangle is special because its side lengths are always in the ratio of 1: √3:2.

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## Do you use sin cos or tan to find the hypotenuse?

But Which One?SOH…Sine: sin(θ) = Opposite / Hypotenuse…CAH…Cosine: cos(θ) = Adjacent / Hypotenuse…TOATangent: tan(θ) = Opposite / Adjacent

## Is tangent sin over COS?

The tangent of x is defined to be its sine divided by its cosine: tan x = sin x cos x . … The secant of x is 1 divided by the cosine of x: sec x = 1 cos x , and the cosecant of x is defined to be 1 divided by the sine of x: csc x = 1 sin x .

## What is the equation for the law of cosines?

The cosine of a right angle is 0, so the law of cosines, c2 = a2 + b2 – 2ab cos C, simplifies to becomes the Pythagorean identity, c2 = a2 + b2, for right triangles which we know is valid.

## How do you find the law of sines?

Simply, it 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 sides and angles in a given triangle. In ΔABC is an oblique triangle with sides a,b and c , then asinA=bsinB=csinC .

## What is the rule of sin?

Sine Rule. The Sine Rule can be used in any triangle (not just right-angled triangles) where a side and its opposite angle are known. You will only ever need two parts of the Sine Rule formula, not all three. You will need to know at least one pair of a side with its opposite angle to use the Sine Rule.

## How do you know if a graph is sine or cosine?

Look at the graphs of the sine and cosine functions on the same coordinate axes, as shown in the following figure. The graph of the cosine is the darker curve; note how it’s shifted to the left of the sine curve. The graphs of y = sin x and y = cos x on the same axes.

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## How do you remember the cosine rule?

You only need to remember the +2abcos(C) bit. Yep. It’s rearranged to resemble Pythagoras’s formula.