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Sine, Cosine, Tangent

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Sine and Cosine Ratios

SOH-CAH-TOA is a mnemonic that many people use to remember the difference between sine, cosine, and tangent. How can remembering SOH-CAH-TOA help you?

Watch This

https://www.youtube.com/watch?v=Jsiy4TxgIME Khan Academy: Basic Trigonometry

Guidance

Two right triangles with one pair of non-right congruent angles are similar by AA \sim . This means the ratio between the side lengths of the first triangle must be congruent to the ratio between the corresponding side lengths of the second triangle. 

For example, in the picture above, \frac{a}{c}=\frac{d}{f} . Because there are three pairs of sides for any triangle, there are three relevant ratios for a given angle.

  1. The tangent of an angle gives the ratio \frac{\text{opposite leg}}{\text{adjacent leg}} . The abbreviation for tangent is tan.
  2. The sine of an angle gives the ratio \frac{\text{opposite leg}}{\text{hypotenuse}} . The abbreviation for sine is sin.
  3. The cosine of an angle gives the ratio \frac{\text{adjacent leg}}{\text{hypotenuse}} . The abbreviation for cosine is cos.

These are the basic trigonometric ratios . Trigonometry is the study of triangles. These ratios are called trigonometric ratios because they apply to triangles. Just as your scientific or graphing calculator has tangent programmed into it, it also has sine and cosine programmed into it. This means that you can use your calculator to determine the ratio between the lengths of any pair of sides for any angle within a right triangle.

Example A

Use your calculator to find the sine ratio and cosine ratio for a  27^\circ angle.

Solution:   \sin (27^\circ) \approx 0.454 and \cos (27^\circ) \approx 0.891 .

Example B

Solve for x .

Solution: Look to see how the sides that are marked are related to the 27^\circ  angle. The side of length 11 is adjacent to the angle. The side of length x  is the hypotenuse of the triangle. When working with the adjacent side and the hypotenuse, you should use the cosine ratio.

\cos (27^\circ) &= \frac{\text{adjacent leg}}{\text{hypothenuse}}\\\cos (27^\circ) &= \frac{11}{x}\\x &= \frac{11}{\cos (27^\circ)}\\x & \approx \frac{11}{0.891}\\x & \approx 12.346

Note that \frac{11}{\cos (27^\circ)}  is the exact answer. 12.346 is an approximate answer because you rounded the value of \cos (27^\circ) .

Example C

Find \sin \theta  and \cos \theta .

Solution: Relative to angle \theta , 3 is the opposite leg and 4 is the adjacent leg. In order to find the sine and cosine ratios you also need to know the hypotenuse of the triangle. Use the Pythagorean Theorem to find the hypotenuse:

3^2+4^2 &= h^2\\25 &= h^2\\h &= 5

Now, write the sine and cosine ratios:

\sin \theta &= \frac{\text{opposite leg}}{\text{hypotenuse}}=\frac{3}{5}\\\cos \theta &= \frac{\text{adjacent leg}}{\text{hypotenuse}}=\frac{4}{5}

Concept Problem Revisited

SOH-CAH-TOA is a mnemonic that many people use to remember the difference between sine, cosine, and tangent. How can remembering SOH-CAH-TOA help you?

SOH-CAH-TOA stands for S ine equals O pposite over H ypotenuse, C osine equals A djacent over H ypotenuse, and T angent equals O pposite over A djacent. This mnemonic helps you remember which trigonometric ratio is which.

Vocabulary

Two figures are similar if a similarity transformation will carry one figure to the other. Similar figures will always have corresponding angles congruent and corresponding sides proportional. 

AA, or Angle-Angle , is a criterion for triangle similarity. The AA criterion for triangle similarity states that if two triangles have two pairs of congruent angles, then the triangles are similar.

The tangent (tan) of an angle within a right triangle is the ratio of the length of the side opposite the angle to the length of the side adjacent to the angle.

The sine (sin) of an angle within a right triangle is the ratio of the length of the side opposite the angle to the length of the hypotenuse.

The cosine (cos) of an angle within a right triangle is the ratio of the length of the side adjacent to the angle to the length of the hypotenuse.

The trigonometric ratios are sine, cosine, and tangent. 

Trigonometry is the study of triangles.

\theta , or “theta”, is a Greek letter. In geometry, it is often used as a variable to represent an unknown angle measure.

Guided Practice

1. Use your calculator to find the sine and cosine ratios for a 39^\circ  angle.

2. Solve for x .

3. Find \sin \theta  and \cos \theta .

Answers:

1.  \sin (39^\circ) \approx 0.629 and \cos (39^\circ) \approx 0.777 .

2. Look to see how the sides that are marked are related to the  39^\circ angle. The side of length 17 is opposite from the angle. The side of length x  is the hypotenuse of the triangle. When working with the opposite side and the hypotenuse, you should use the sine ratio.

\sin (39^\circ) &= \frac{\text{opposite leg}}{\text{hypotenuse}}\\\sin (39^\circ) &= \frac{17}{x}\\x &= \frac{17}{\sin (39^\circ)}\\x & \approx \frac{11}{0.629}\\x & \approx 27.013

3. Relative to angle \theta , 4 is the opposite leg and 2 is the adjacent leg. In order to find the sine and cosine ratios you also need to know the hypotenuse of the triangle. Use the Pythagorean Theorem to find the hypotenuse:

4^2+2^2 &= h^2\\20 &= h^2\\h &= 2 \sqrt{5}

Now, write the sine and cosine ratios:

\sin \theta &= \frac{\text{opposite leg}}{\text{hypotenuse}}=\frac{4}{2 \sqrt{5}}=\frac{2}{\sqrt{5}}=\frac{2 \sqrt{5}}{5}\\\cos \theta &= \frac{\text{adjacent leg}}{\text{hypotenuse}}=\frac{2}{2 \sqrt{5}}=\frac{1}{\sqrt{5}}=\frac{\sqrt{5}}{5}

Note that in the last step of each calculation the denominator was rationalized. You can choose to rationalize the denominator if you wish.

Practice

For #1-#6, use the triangle below. Find each exact value.

1.  \sin E

2.  \cos E

3.  \tan E

4.  \sin F

5.  \cos F

6.  \tan F

Identify whether the sine, cosine, or tangent ratio is most useful for helping to solve the problem. Then, solve for x .

7.

8.

9.

10.

11.

12.

Use the triangle below for #13-#15.

13. Find m \angle C .

14. Draw an altitude from \angle B  to divide the triangle into two right triangles. Use trigonometry to find the lengths of the sides of each of these right triangles.

15. Find the perimeter of \Delta ABC .

16. True or false: You need to know the length of at least one side of a triangle to find the lengths of the other sides.

17. What are the trigonometric ratios?

18. What do the trigonometric ratios have to do with similar triangles?

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