Geometry and Trigonometry practice question

Answer: 0.8 or 4/5

For any angle, sin²θ + cos²θ = 1. So cos²θ = 1 − 0.36 = 0.64, and since θ is acute its cosine is positive: cos θ = 0.8. (This is the 3-4-5 triangle: sides 3, 4, 5 give sine 3/5 and cosine 4/5.)

Common wrong answers

If you answered 0.4

0.4 is 1 − 0.6. The identity involves the SQUARES: 1 − 0.6² = 0.64, and then take the square root.

If you answered 0.36

0.36 is sin²θ. You need 1 − 0.36 = 0.64 for cos²θ, then the square root, 0.8.

If you answered 0.64

0.64 is cos²θ, the cosine SQUARED. The cosine itself is √0.64 = 0.8.

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