Question

Which triangle is it ?

Answer 1

The answer is: The first triangle. The reasons are shown below:

 1. All the triangles are rigth triangles, because they have an angle of 90°. So, let's calculate the others angles of the first one:

 Tan(α)^-1= opposite leg/adjacent leg

 Opposite leg=5
 Adjacent leg=5√3

 Tan(α)^-1= 5/5√3
 Tan(α)^-1=30°

 2. Let's calculate the other angle:

 Tan(α)^-1= opposite leg/adjacent leg

 Now, the opposite leg will be 5√3 and the adjacent leg will be 5. Then:

 Tan(α)^-1= 5√3/5
 Tan(α)^-1=60°

 As you can see, the angles of first triangle are: 30°,60° and 90°.