The Cosine Ratio
Part of the Right-Triangle Trigonometry section of Coddy's Trigonometry journey. Lesson 8 of 50.
The cosine of θ uses the side next to θ instead: the adjacent side divided by the hypotenuse.
cos θ = adjacent / hypotenuse
In the 3, 4, 5 triangle with θ opposite the 3, the adjacent side is 4, so cos(theta) = 4/5. Look from the other acute angle and the side of 4 is opposite, so the other angle's sine is also 4/5. That is where the name comes from: cosine is the sine of the complement, the angle that makes 90° with θ. So cos(60°) is sin(30°), which is 1/2.
Challenge
Triangle KLM has a right angle at M and a 60° angle at K. The hypotenuse is 18. Find the side x next to the 60° angle: tap 60°, x and 18, pick the ratio, and solve. Then press Check.
Try it yourself
Use the figure to find x
Test yourselfThis 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 HypotenuseThe Opposite SideThe Adjacent SideWhen the Angle MovesRecap: Naming the Sides2The Three Ratios
The Sine RatioThe Cosine RatioThe Tangent RatioChoosing the RatioRecap: The Three Ratios