Help with sine/cosine(?) rule (1 Viewer)

[guy123]

I don't even know how to begin, can I assume that the right angle is 90 degrees? But then my answer doesn't correspond with the correct answer, which is A

 

[Carrotsticks]

The largest angle in a triangle is opposite the longest length.

So clearly, the largest angle is the one opposite 3.5 or the 'hypotenuse'.

So using the cosine rule, we have:

 

[powlmao]

You cannot assume as the angle you are talking about isn't a right angle

CosC = (a^2 + b^2 - c^2)/2ab

Angle c is the angle you thought was a right abgle

 

[D94]

Well, the biggest angle must be located between the 2 shortest sides. Then use the cosine rule to determine that angle. There are no right angles in that given triangle, hence the "not to scale" part.

 

[guy123]

Sorry for the dumb questions but I've had them piling up and I got a new teacher last term and she was terrible. You guys are explaining much better than her, thanks for your help

 

[Carrotsticks]

Sorry for the dumb questions but I've had them piling up and I got a new teacher last term and she was terrible. You guys are explaining much better than her, thanks for your help

No need to apologise.

Ask us as many questions as you like, no matter how trivial they may seem.

Provided you are polite when asking and show some sort of appreciation afterwards, all of us here will be more than happy to answer any of your questions =)

 

[Drongoski]

2 easier methods(for multiple choice workings not required):

1) draw a triangle with sides 5, 6 and 7 (double the actual lengths) and measure the largest angle (facing the side 7). You will find it is about 78 deg.

2) 5² + 6 ² = 61 > 49 = 7²

.: cannot be right-angled, in fact largest angle must be less than 90 deg. But since 61 is way larger than 49, it is nowhere near a right angle, therefore cannot be 89 deg.

Like my approach? No need sine/cosine rules.

 

[D94]

Like my approach? No need sine/cosine rules.

This method is purely based on the MC available. If they were closer in value, you would need to use the sine/cosine rules. But then again, one could postulate the argument that since 2.5~3~3.5, the angles would need to hover around 60 degrees, so 78 degrees is the best bet. Again, based on the MC available.

 

[Drongoski]

This method is purely based on the MC available.

Of course I took full advantage of the given situation. Wasn't claiming if m.c. were 1) 77.8 2) 77.9 3) 78.1 4) 78.7 that I could have done it this way nor would I.