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Grade 9:  The Learning Equation Math

31.03: Finding Unknown Angles

 Refresher pp 62-63

 SeekAWord2ech.class Author Prerequisite Skills:

Review skills from first 2 lessons:

Ratios in Right Triangles:  Grade 9 Lesson 31.01

Ratios in Right Triangles:  Grade 9 Lesson 31.02

 acute, angle, trigonometry, right triangle, sine, cosine, tangent, hypotenuse, opposite side, adjacent side Note: acute and angle need to be added to this puzzle.

Learning Outcomes:

The student will:

## Sum of Three Angles in a Triangle is 180o

If you know the sum of the angles of any triangle is 180o, you do not need trigonometry functions to calculate the size of an angle when the other two angles are known.  The following applet will help you visualize an important triangle calculation tool:

## Sum of Angles in a Triangle

The following animation shows why the sum of the angles in any triangle is 180°. Remember to click [Go] to move to next animation.

## Practice

The "Introduction" reviews the Pythagorean theorem. "Naming Sides" reviews the trigonometry triangle names required to use "Soh Cah Toa". The "Practice" section has 5 sections of problem types. For this activity complete problem types 10 - 12. The activity below this one focuses on the same 3 problem types.

• Problem Types 1-3: Pythagorean Theorem.
• Problem Types 4-6: Calculate the sine, cosine and tangent ratios given one reference angle.
• Problem Types 7-9: Calculate the sine, cosine and tangent ratios given two appropriate sides.
• Problem Types 10-12: Calculate the reference angle given two appropriate sides.
• Problem Types 13-18: Calculate the length of one of the sides given appropriate side and reference angle.

Using Trigonometry to Calculate Angles

If we know the length of two sides of a right triangle ABC we can calculate the size of either acute angle using one of the trigonometry functions.  The buttons below show the sides and trigonometry function you will need to calculate an angle.  Click each "A" button to learn how to calculate the angle using our 3 trigonometry functions.

Click "New Problem" to create sample problems to find an unknown angle from the lengths of two sides of a right triangle.



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Original source:  Pythagoras and Trigonometry

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Review:

Triangles also have sides.  If you know the length of two sides of a right triangle, you do not need to use trigonometry functions.

You can calculate the length of the third using the Pythagorean Theorem.

## a2 + b2 = c2

Pythagoras Theorem - interactive proof

History of Pythagoras

 In the second part of the last lesson helped you learn how to calculate side lengths using the various trigonometry functions when one side and one acute angle are known.  Click a button to see examples of how to calculate that side using a trigonometry function. Sine ratio S = o/h Cosine ratio C = a/h Tangent ratio T = o/a / / /

 sinD = 4/5

 Trigonometry Calculator The following site will show you how to use the trigonometry ratios to calculate the lengths of a side in in a right triangle.  Right Angle Relationships To use: Fill in a value for 1 of the 3 sides, fill in a value for angle D or E. put a question mark,  (?), in the box of the side whose length you are trying to find, then click "Go" to see how to find the measure of your angle. An example has been completed for you.  Remove these and substitute your own values to investigate right angle triangles further.

Quia! Challenge Board - Introductory Trigonometry

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