The only prerequisite to learning trigonometry is knowing the **Pythagorean theorem**. If you’d like to touch up on the *Pythagorean theorem*, review this video link for clarification.

- Feel free to skip it if you’re comfortable with the concept.

The reason it’s important is because the theorem introduces you to two key terms: **right triangle** (triangle containing a 90° angle), and **hypotenuse** (the longest side of a *right triangle*, opposite of the right angle). An animated illustration of a right triangle is provided below:

**Trigonometric functions **are **ratios** (comparisons) of opposing side lengths relative to a **reference angle** of your choosing. The reference angle can be any of the two *acute angles* (*angles less than *90º) within the right triangle. Depending on which one you choose, the ratio will change. This is also illustrated in the animation above. Notice that when **∠A (angle A)** is chosen, *side a* is labelled opposite and *side b* adjacent. Similarly, when **∠B** is chosen, , *side b* is labelled opposite and *side a* adjacent. The first trigonometric function we’ll focus on is **tangent**.

## The Tangent Ratio

The tangent ratio is a comparison of the opposite length relative to the reference angle of your choosing compared to the adjacent length. Notice in the animation that the **acute angles** are 30° and 60º. The side *opposite* of 30° is y and *adjacent* is x. Whereas if we chose 60º as our acute angle, the *opposite* length is x and *adjacent* is y. That’s summarized below:

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In case you forget, the tangent ratio can be remembered as **T**_{angent}**O**_{pposite}**A**_{djacent} (TOA).

## The Sine Ratio

What if we wanted to compare the ** opposite **to

**length? In that case, we’d use another trigonometric ratio known as**

*hypotenuse***Sine**. Let’s investigate sine using the generic right triangle below:

Notice the symbol (θ, “theta”). That will be considered our **reference angle **here. As mentioned before, the reference angle can be any of the acute angles (less than 90°) in the right triangle – **never** choose the 90° angle as your reference. Therefore, the sine ratio applied to θ is summarized below:

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The sine ratio can be remembered as **S**_{ine}**O**_{pposite}**H**_{potenuse} (SOH).

## The Cosine Ratio

The last comparison we’ll make using the same right triangle shown above is the **cosine ratio**. The cosine ratio is a comparison of the **adjacent length** to the **hypotenuse** relative to the angle θ:

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The sine ratio can be remembered as **C**_{osine}**A**_{djacent}**H**_{potenuse} (CAH). Collectively, the **sine**, **cosine**, and **tangent ratios** are referred to as **trigonometric ratios**. Using all three abbreviations stated above, the best way to remember the ratios is through the mnemonic:

**SOH CAH** **TOA**

Trigonometric ratios can be used to find two main things:

**Missing sides**in a right triangle**Missing angles**in a right triangle

To find the length of an unknown side, you must be given **one acute reference angle** and **one known side**. Then you have a straight forward calculation involving your calculator and some algebraic manipulation. Keep in mind that most modern day calculators come **pre-programmed** with all possible ratios of sine, cosine, and tangent for any angle. So if you wanted to find the ratio for **sin(50°)**, you’d click the sine button (sine function), then your angle (shown in yellow):

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Try it yourself, and make sure your calculator is in **degree mode** (shown in red). Notice that four numbers after the decimal place were kept for good measure. This will ensure that if you do the opposite – where you want to find an angle given the ratio – you get an angle that’s accurate to 50° (this will be discussed further below). Also, by writing **0.7660 over 1**, it illustrates how sine of 50 degrees is comparing the opposite length (0.7660) to the hypotenuse (1).

This leads us to the second reason we use trigonometric ratios. As mentioned in point (2), if we have **2 known sides**, the unknown acute angle of any of the vertices can be found using **inverse trigonometric functions** that are also found on your calculator (blue arrow). You’ll have to click “shift” or “2nd” first to access these functions. The two videos below will walk you through four different examples related to finding the angle when only the sides are known.

Summary of inverse trigonometric functions:null

Of course, this lesson wouldn’t be complete if we didn’t see examples where we find an **unknown side** when given a known **acute angle** and a **known side**.

Summary:The three primary trigonometric ratios are sine, cosine, and tangent. They are defined as follows: null

You can find any side length or angle measure of a right triangle if you know

two pieces of informationin addition to the right angle.