# Some rule v cosine rule

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#1
For jun 10 Oct mei 11a
Why use some rule to find the angle
Why not cosine
I used cosine and got a value like 54.8 when it should be 50
How do I know when to use some rule and cosine rule for angles?
0
3 years ago
#2
'Some rule'...?

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PLEASE don't let this refer to the SINE rule 1
3 years ago
#3
u label the sides with opposite, adjacent, hypotenuse and the 2 which aren't missing can determine which rule u have to use
0
3 years ago
#4
(Original post by esmeralda123)
For jun 10 Oct mei 11a
Why use some rule to find the angle
Why not cosine
I used cosine and got a value like 54.8 when it should be 50
How do I know when to use some rule and cosine rule for angles?
Assuming you mean "sine rule", both the sine and cosine rules should give the same answer since it's the same angle. You use sine rule when you're looking at a pair of a side and the angle opposite it. You use cosine rule when you have all 3 sides and you want an angle. Your error probably came from rounding errors or just a miscalculation.

If you have all the information you need, cosine rule is more reliable because of the ambiguous case for sine.
0
3 years ago
#5
(Original post by esmeralda123)
For jun 10 Oct mei 11a
Why use some rule to find the angle
Why not cosine
I used cosine and got a value like 54.8 when it should be 50
How do I know when to use some rule and cosine rule for angles?
If you have three pieces of information (a piece is either a side length or angle), look at those three plus the one you are trying to find out. If two sides and two angles are involved use sine rule; if three sides and three angles are involved use cosine rule (and if there are three angles and one side, it's impossible if you're trying to find the side, and if you're trying to find the angle use sum of angles = 180 degrees). If you have four or more pieces of information, you will probably have a choice so do whichever rule you find easiest (personally I would do cosine rule for if you're finding a side and have the angle opposite it and sine rule otherwise).
0
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