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# Trig help watch

1. I understand how to use them, but can someone clearly explain to me when to use all the different formulas in Trigonometry? (For e.g: how I can tell when to use Cosine rule)
2. (Original post by chicchoc)
I understand how to use them, but can someone clearly explain to me when to use all the different formulas in Trigonometry? (For e.g: how I can tell when to use Cosine rule)
Cosine rule:
2 sides + enclosed angle to find opposite side
All 3 sides to find an angle

Sine rule:
Angle and opposite side + side to find the other opposite angle
Angle and opposite side + angle to find the other opposite side

Sine area rule:
To find area you need 2 sides + enclosed angle
To find a side you need the area + a side and the enclosed angle

In right angled triangles:
Sine:
To find angle you need opposite side + hypotenuse
To find hypotenuse you need angle + opposite side
To find opposite side you need angle + hypotenuse

Cosine:
To find angle you need adjacent side + hypotenuse
To find hypotenuse you need angle + adjacent side
To find adjacent side you need angle + hypotenuse

Tangent:
To find angle you need opposite side + adjacent side
To find adjacent side you need angle + opposite side
To find opposite side you need angle + adjacent side

3. My revision guide described the trig rules quite well; I can post a photo of the explanation.
4. (Original post by Loci Pi)
Cosine rule:
2 sides + enclosed angle to find opposite side
or 1 side, angle, and side opposite to the angle to find other side enclosing the angle.

Sine rule:
Angle and opposite side + side to find the other opposite angle
Angle and opposite side + angle to find the other opposite side

Sine area rule:
To find area you need 2 sides + enclosed angle
To find a side you need the area + a side and the enclosed angle

In right angled triangles:
Sine:
To find angle you need opposite side + hypotenuse
To find hypotenuse you need angle + opposite side
To find opposite side you need angle + hypotenuse

Cosine:
To find angle you need adjacent side + hypotenuse
To find hypotenuse you need angle + adjacent side
To find adjacent side you need angle + hypotenuse

Tangent:
To find angle you need opposite side + adjacent side
To find adjacent side you need angle + opposite side
To find opposite side you need angle + adjacent side

Not completely correct -

Cosine rule:
Three sides - use to get largest angle [oppostie largest side]
Are you sure about the seond use of cosine rule - draw a diagram as it doesn't sound correct.
5. (Original post by Muttley79)
Not completely correct -

Cosine rule:
Three sides - use to get largest angle [oppostie largest side]
Are you sure about the seond use of cosine rule - draw a diagram as it doesn't sound correct.
You're right sorry I made a stupid mistake explaining it I will edit my comment.
6. TSR won't let me upload the photo for some reason.
7. (Original post by chicchoc)
I understand how to use them, but can someone clearly explain to me when to use all the different formulas in Trigonometry? (For e.g: how I can tell when to use Cosine rule)
The second post in the thread was incorrect but has now been amended.
8. I have corrected the mistake now. The rest of it is correct though.

9. Sine rule

10. Cosine rule

11. Sine area rule

You can ignore the fact that 'sine' is highlighted.

12. Right angled triangles
13. do you have any for c3 and c4 trig?
14. (Original post by RuthieG101)
do you have any for c3 and c4 trig?
No, sorry. The most trig I know is C2 AS level but when I come to learning C3 and C4 diagrams like that would be useful.

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Updated: March 5, 2018
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