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Solving trigonometric equations using pythagorean identities

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Grade 12 Advanced Functions, University Preparation (MHF4U)
B. Trigonometric Functions
25. 12AF.B3.4
25.4 Solving trigonometric equations using pythagorean identities

Solving Trigonometric Equations Using Pythagorean Identities

Turn a mixed trig equation into a simple factorable one by swapping in a Pythagorean identity, then solve it step by step.


What You'll Learn

A Pythagorean identity lets you replace one squared trig function with an equivalent expression in another, so a mixed equation becomes a single-function equation.
The three key identities are sine squared plus cosine squared equals 1, 1 plus tangent squared equals secant squared, and 1 plus cotangent squared equals cosecant squared.
After substituting, the equation usually becomes a quadratic in one trig function that can be solved by factoring.
Reference angles and the ASTC rule are used to turn each trig value back into actual angle solutions.
Always check the given interval and any excluded values, since squaring or substituting can introduce solutions that don't actually work.

What You'll Practice

1

Converting mixed sine and cosine equations using sin²x + cos²x = 1

2

Factoring trigonometric trinomials after simplification

3

Finding all angle solutions when sine or cosine equals specific values

4

Using special triangles and the unit circle to determine reference angles

Why This Matters

Solving trigonometric equations using Pythagorean identities is essential for physics, engineering, and calculus. You'll use these skills to analyze wave motion, model periodic phenomena, and solve complex problems in advanced mathematics courses.

This Unit Includes

1 Video lesson
Practice exercises
Learning resources

Skills

Pythagorean Identities
Trigonometric Equations
Factoring
Unit Circle
Special Triangles
Quadrant Analysis
Algebraic Substitution
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