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# (M4)Geometry gone wrong? watch

1. The question here really isn't important, it's more the solution, but I'll post the question anyways:

This isn't the whole solution, I cut out a bit that isn't needed:
Attachment 514881514883

The problem is with triangle , in the second last line, they said , which is what I got when I did this question. But then if you use the cosine rule to fine AB, you don't get the same answer. I used an online calculator just to make sure.
Attachment 514881514883514900

Can someone tell me what's going on here? I mean "sensically" their answer is the correct one, but I don't see how it actually is. I mean why are they claiming that ABC is isosceles? I just can't figure out what's going on here.

Thanks!
Attached Images

2. (Original post by gagafacea1)
The question here really isn't important, it's more the solution, but I'll post the question anyways:

This isn't the whole solution, I cut out a bit that isn't needed:
Attachment 514881514883

The problem is with triangle , in the second last line, they said , which is what I got when I did this question. But then if you use the cosine rule to fine AB, you don't get the same answer. I used an online calculator just to make sure.
Attachment 514881514883514900

Can someone tell me what's going on here? I mean "sensically" their answer is the correct one, but I don't see how it actually is. I mean why are they claiming that ABC is isosceles? I just can't figure out what's going on here.

Thanks!
The triangle in the calculator is not triangle ABC. That triangle doesn't exist anywhere.

They worked out angle ACB to be 112.5 degrees. ABC is an isosceles therefore CAB and CBA are both 33.75 degrees. You now have all 3 angles but no sides. To find the magnitude of AB you need at least 1 side. Either S1b or S2A and then use the bigger triangle S1BA or S2AB to find the magnitude of AB.
3. (Original post by AlmostNotable)
The triangle in the calculator is not triangle ABC. That triangle doesn't exist anywhere.

They worked out angle ACB to be 112.5 degrees. ABC is an isosceles therefore CAB and CBA are both 33.75 degrees. You now have all 3 angles but no sides. To find the magnitude of AB you need at least 1 side. Either S1b or S2A and then use the bigger triangle S1BA or S2AB to find the magnitude of AB.
The triangle in the calculator is S1BA , not ABC.
4. (Original post by gagafacea1)
The triangle in the calculator is S1BA , not ABC.
No it isn't. For it to be S1BA the angle on the right needs to be 33.75.
5. (Original post by AlmostNotable)
No it isn't. For it to be S1BA the angle on the right needs to be 33.75.
Yes that's my point, if the angle on the left is 22.5 and the sides adjacent to it are 21.648 (S1B) and 40 (S1A), the angle on the right cannot be 33.75.
6. (Original post by gagafacea1)
Yes that's my point, if the angle on the left is 22.5 and the sides adjacent to it are 21.648 (S1B) and 40 (S1A), the angle on the right cannot be 33.75.
I'll do the question and tell you what I get.
7. AB=S1B=21.6
Angle AS1B=S1AB=22.5 degrees
8. (Original post by AlmostNotable)
AB=S1B=21.6
Angle AS1B=S1AB=22.5 degrees
That's what I got two. I just don't know what they're doing.

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