# Discover the relationship of trigonometric ratios for similar triangles

### Triangle Trigonometry

All you need to understand is geometric notion of similar triangles: This gives us our connection between triangle trigonometry and the trigonometric functions trig functions (previous pop-up), then you know that they are just ratios of the. Determine the six trigonometric ratios for a given angle in a right triangle. · Recognize the reciprocal relationship between sine/cosecant, cosine/secant, and Use a calculator to find the value of the six trigonometric functions for any . This ratio will be the same for all similar triangles, and this ratio is called the sine of 35°. How are trigonometric ratios derived from the properties of similar triangles? Students explore the relationship between the sine and cosine of complementary angles. Challenge students to find a way to determine the height of a tree.

So if this angle is theta, this is They all have to add up to degrees. That means that this angle plus this angle up here have to add up to We've already used up 90 right over here, so angle A and angle B need to be complements.

So this angle right over here needs to be 90 minus theta.

## Trigonometric ratios in right triangles

Well we could use the same logic over here. We already use of 90 degrees over here. So we have a remaining 90 degrees between theta and that angle.

So this angle is going to be 90 degrees minus theta. You have three corresponding angles being congruent. You are dealing with similar triangles. Now why is that interesting?

### Triangle similarity & the trigonometric ratios (video) | Khan Academy

Well we know from geometry that the ratio of corresponding sides of similar triangles are always going to be the same. So let's explore the corresponding sides here. Well, the side that jumps out-- when you're dealing with the right triangles-- the most is always the hypotenuse.

So this right over here is the hypotenuse. This hypotenuse is going to correspond to this hypotenuse right over here. And then we could write that down.

This is the hypotenuse of this triangle.

This is the hypotenuse of that triangle. Now this side right over here, side BC, what side does that correspond to? Well if you look at this triangle, you can view it as the side that is opposite this angle theta. If you go across the triangle, you get there.

So let's go opposite angle D. If you go opposite angle A, you get to BC. Opposite angle D, you get to EF.

So it corresponds to this side right over here.

And then finally, side AC is the one remaining one. We could view it as, well, there's two sides that make up this angle A.

### Introducing Trig | nzmaths

One of them is the hypotenuse. We could call this, maybe, the adjacent side to it. And so D corresponds to A, and so this would be the side that corresponds.

Now the whole reason I did that is to leverage that, corresponding sides, the ratio between corresponding sides of similar triangles, is always going to be the same. These are similar triangles. They're corresponding to each other. And we could keep going, but I'll just do another one. And we got all of this from the fact that these are similar triangles.

So this is true for any right triangle that has an angle theta.

## Discovering Trigonometric Ratios

Then those two triangles are going to be similar, and all of these ratios are going to be the same. Well, maybe we can give names to these ratios relative to the angle theta. So from angle theta's point of view-- I'll write theta right over here, or we can just remember that-- what is the ratio of these two sides?

Well from theta's point of view, that blue side is the opposite side.

It's opposite-- so the opposite side of the right triangle. And then the orange side we've already labeled the hypotenuse. So from theta's point of view, this is the opposite side over the hypotenuse. The altitude of a triangle is the geometric mean of the segments of the hypotenuse that it divides. Its side lengths form a very special ratio which must be memorized.

**Similarity to define sine, cosine, and tangent - Basic trigonometry - Trigonometry - Khan Academy**

Specifically, if the legs are both of length x, then the hypotenuse is of length x by the pythagorean theorem. One way is to start with an equilateral triangle, bisect one angle which also bisects the side opposite, and consider the resulting congruent triangles.

Again, by the pythagorean theorem, the side length ratios can be found to be 1: By the AA Similarity Theorem, any triangle with these angles has these exact side length ratios. In right triangles, the six side length ratios are proportional to the angles and named as follows.

Another one only recently brought to my attention is: Note how the right hand column reciprocates the left hand column. For this reason, the left hand column are considered the primary trigonometric functions and the right hand column secondary. Most calculators only have the primary trig functions. In this case we often label one leg y and the other leg x. Consider what happens as we let the angle at the origin change.

As the angle increases, the side opposite increases. Tangent is not only the ratio of the opposite side to the adjacent side, but can also be written as sine over cosine. Large tables of trig functions were commonplace before calculators became ubiquitous. Notice how the cosine values are the same as the sine values of the complementary angle.

Tangent and cotangent are similarly related.