Actually, Ed0gawa was correct when he used 'is concerned'. While "concerned" is past tense, "is concerned" is present tense.
Valkyrie's sentence is incorrect, however, although I'm struggling to explain why ^^' Probably because I should be asleep right now. What I can tell you is that just because you are talking about something in the present, doesn't mean that absolutely everything in the sentence must be in present tense.
"Right-angled triangle" always remains that way, no matter what tense the rest of the sentence is in. "Right-angled" is the description of the triangle.
Eg.
Sarah crouched down to pick up the folded piece of paper.
This sentence is being told in past tense, so why did I not write "picked up"?
If we break this sentence down a little bit, it becomes:
1)Sarah crouched down
2)to pick up
3)the folded piece of paper.
The verb in phrase 1 is the one that describes what the subject, Sara, is doing. This verb tells us what tense the sentence is in.
The verb in phrase 2 describes what Sara is doing to the piece of paper in phrase 3.
The verb in phrase 3 is merely describing the piece of paper.
You could also change the sentence around to make it present tense or future tense.
Sarah crouches down to pick up the folded piece of paper.
Sarah will crouch down to pick up the folded piece of paper.