Which of the following triangles is a $30°$-$60°$-$90°$ triangle?

• A. A triangle with angles $30°$, $60°$, and $90°$
• B. A triangle with angles $60°$, $60°$, and $60°$
• C. A triangle with angles $20°$, $70°$, and $90°$
• D. A triangle with angles $45°$, $45°$, and $90°$

Answer: (A)

Verified step by step guidance

1. Recall that a 30°-60°-90° triangle is a special right triangle with angles exactly 30 degrees, 60 degrees, and 90 degrees.
2. Check each given triangle's angles to see if they match these three specific angle measures.
3. The first option has angles 30°, 60°, and 90°, which matches the definition of a 30°-60°-90° triangle.
4. The second option has three 60° angles, which forms an equilateral triangle, not a 30°-60°-90° triangle.
5. The third and fourth options have angles that do not match the 30°, 60°, and 90° combination, so they are not 30°-60°-90° triangles.