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When the Angle Moves

Part of the Right-Triangle Trigonometry section of Coddy's Trigonometry journey. Lesson 5 of 50.

Opposite and adjacent are not fixed names: they depend on which acute angle you stand at. Move to the other acute corner and the two shorter sides swap names. Only the hypotenuse keeps its name.

$Right triangle ABC with sides 3, 4 and 5, angle α at A and β at B$

In this triangle, stand at α: the side of 3 is opposite and the side of 4 is adjacent. Stand at β: now the side of 4 is opposite and the side of 3 is adjacent. The side of 5 is the hypotenuse either way. So before you name a side, check which angle the question is about.

Challenge

Triangle KLM has a right angle at M. This time θ is at L. Tap the side opposite θ. Then press Check.

Try it yourself

$Find the right part on the figure$

Find the right part on the figure

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Right-Triangle Trigonometry

1Sides of a Right Triangle

How Tall Is the Tree?The Hypotenuse The Opposite Side The Adjacent SideWhen the Angle MovesRecap: Naming the Sides

4Finding the Hypotenuse

The Unknown Below the Bar How Long Is the Kite String?A Leg Below the Bar Is the Answer Sensible?Recap: The Hypotenuse

7Elevation and Depression

Angle of Elevation Measuring From Eye Level Angle of Depression The Sun's Angle Two Triangles, One Height Recap: Looking Up and Down

2The Three Ratios

The Sine Ratio The Cosine Ratio The Tangent Ratio Choosing the Ratio Recap: The Three Ratios

5Finding an Angle

Inverse Sine Inverse Cosine Inverse Tangent The Angle of a Ramp Both Acute Angles Recap: Finding an Angle

8Reciprocal Ratios

Cosecant, Secant, Cotangent Exact Reciprocal Values A Side From a Cosecant A Ladder and a Secant Recap: Reciprocal Ratios

3Finding a Side

Sides at Any Angle Rounding the Answer Tangent: Leg From Leg A Ladder Against a Wall The Other Acute Angle Recap: Finding a Side

6Special Triangles, Exactly

The Two Special Triangles An Exact Hypotenuse The Side Next to 30°An Exact Angle Recap: Exact Values

9Final Challenges

Is the Ladder Safe?The Wheelchair Ramp Two Observers The Landing Plane The Kite The Sun's Angle, Exactly