Math Expression: Acute Triangle Practice Question

Which of the following triangles are acute triangles?

Step 1

For triangle I, all the internal angles of this triangle are <90^°. Hence, it is an acute triangle.
Step 2

As for triangle II, one of the internal angle is 90°. Since this angle is not <90^°, it is not an acute triangle.
Step 3

Now, for triangle III, one of the internal angle is 111° which is not <90^°. Hence, it is not an acute triangle.
Step 4

For triangle IV, all the internal angles are <90^°. Hence, it is an acute triangle.
Step 5

Clearly, only triangle I and IV are acute triangles. Therefore, the answer is D.