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# Maths year 11 Watch

1. (Original post by z_o_e)
Wait. I can't use trigonometry here to find the angle as it isn't a right angled triangle?

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You still use trigonometry, using trigonometry doesn't mean it has to be a right-angled triangle. You use the cosine rule here, and cosine is a trigonometric function.
2. (Original post by RDKGames)
You still use trigonometry, using trigonometry doesn't mean it has to be a right-angled triangle. You use the cosine rule here, and cosine is a trigonometric function.
But I don't know how to find an angle using the trigonometry?

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3. (Original post by z_o_e)
But I don't know how to find an angle using the trigonometry?

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Start with the formula, label the wanted angle as whatever you want, and write out the things you know from that triangle.
4. (Original post by RDKGames)
Start with the formula, label the wanted angle as whatever you want, and write out the things you know from that triangle.
I don't get that. Because that formulae is for missing sides of a triangle?

And this has all its sides...

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5. (Original post by z_o_e)
I don't get that. Because that formulae is for missing sides of a triangle?

And this has all its sides...

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The cosine rule can be used to work out the angle as well by rearranging it for cosine. Just write out the info first of what you know.
6. (Original post by RDKGames)
The cosine rule can be used to work out the angle as well by rearranging it for cosine. Just write out the info first of what you know.

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7. Now plug it through the cosine rule with an unknown angle.
8. (Original post by RDKGames)
Now plug it through the cosine rule with an unknown angle.

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9. That's correct. Now you can do the inverse cosine of both sides for the angle.
10. (Original post by RDKGames)
That's correct. Now you can do the inverse cosine of both sides for the angle.
Didn't get that..

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11. (Original post by z_o_e)
Didn't get that..

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Try again. You entered something on your calculator wrong then.
12. (Original post by RDKGames)
Try again. You entered something on your calculator wrong then.

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13. Correct

But angles aren't measured in centimetres.
14. (Original post by RDKGames)
Correct

But angles aren't measured in centimetres.
Woowowe thank you ♥

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15. (Original post by z_o_e)
Woowowe thank you ♥

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16. (Original post by RDKGames)
So question one is asking for the angles of a and b added up?

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17. (Original post by z_o_e)
So question one is asking for the angles of a and b added up?

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No. This is a recurring misinterpretation for you. Angle ABC means the angle created by the lines AB and BC.

A, B and C are vertices of the polygon, they are not angles.
18. (Original post by RDKGames)
No. This is a recurring misinterpretation for you. Angle ABC means the angle created by the lines AB and BC.

A, B and C are vertices of the polygon, they are not angles.
So what am I suppose to find?

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19. (Original post by z_o_e)
So what am I suppose to find?

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The acute angle at B.
20. (Original post by RDKGames)
The acute angle at B.
I don't think I'm ready for this question just yet. Maybe tomorrow? I can't understand these questions for some REASON ABC

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Updated: December 10, 2016
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