# 3-4-5 triangle

1. Mar 12, 2007

1. The problem statement, all variables and given/known data

Hey, I have quick question. I tried looking on google, but couldn't quite reassure myself.

Does a 3-4-5 triangle have to be a 30-45-90 triangle? Can the angles be any angle?(this is what I think....but not confident).

I have a triangle(this is actually a physics problem)....but the triangle has a leg which is 3' and a leg that is 4'.....so the hyp must be 5'....but i need to know the angles.

2. Relevant equations

3. The attempt at a solution

2. Mar 12, 2007

### robphy

No, a 3-4-5 triangle does not have a 30 or 45 degree angle.
No.

what is the definition of the trigonometric functions: sine, cosine, and tangent?

3. Mar 12, 2007

### ZioX

There exists no triangle with 30-45-90 angles. The sum of the angles must be 180.

It should be noted that the majority of sine, cosine and tangent arguments only work for right angled triangles.

It's not necessarily true that a RIGHT triangle with 3 inch and a 4 inch lengths will have another side of length 5. How do you know that 4 isn't the hypoteneuse?

4. Mar 12, 2007

### AlephZero

The angles of a triangle add up to 180 degrees, so a 30-45-90 triangle is impossible.

No, if you have the length of all 3 sides, the 3 angles are fixed.

You can find the angles of any shape of triangle using the cosine rule, if you know all three sides. For a 3-4-5 triangle, you know one angle is right angle, so you can save time and use the definitions of sine and cosine instead of using the full cosine rule.

5. Mar 12, 2007

I have a picture......I guess I'm having a brain fart.....

I know there is a 90 deg angle(where the 3 and 4 sides meet,angle opposite from the hyp).....so then, can't i use the law of sines to find the angles?

6. Mar 12, 2007

### robphy

What specifically is equal to "opp/adj"? etc...

7. Mar 12, 2007

ok......so can't I just do this.....

sin90/5 = sinx/3 which I think gives me a 36.87 deg angle. This is what I came up with in the begining....just didn't seem right.

8. Mar 12, 2007

tan = opp/adj.......or I think its also sin/cos(or something like that.....y/x?).....definately know its tan.

9. Mar 12, 2007

### AlephZero

Yes, that's yet another way to do it if you don't like the other suggestions.

10. Mar 12, 2007