The Sine Rule

Question 1

When may we need the sine rule or cosine rule?

Answer 1

We may need the sine rule or cosine rule if the question contains lengths or angles in a triangle.

Question 2

How do we know to use the sine rule?

Answer 2

If the triangle is not right-angled you may need the sine rule or the cosine rule.

Question 3

What is the formula for the sine rule?

Answer 3

The formula for the sine rule is:
In any triangle ABC: a/sinA=b/sinB=c/sinC
SinA/a = sinB/b = sinC/c

(Where capitals are angles and lower case are sides)

(Formula sheet)

Question 4

Only when can we use the sine rule?

Answer 4

We can only use the sine rule if we have a matching pair. We can find a missing value from another pair.

Question 5

What should we always look for when using the sine rule?

Answer 5

When using the sine rule we should always look for PAIRS

Question 6

What is our thinking process when we are deciding whether to use the sine rule or not?

Answer 6

Is it a right-angled triangle? No
Is there a matching pair? Yes
Use the sine rule.
When we are trying to find the missing length of a side, the small letters are on the top. (a/sinA = b/sinB)

When we are trying to find a missing angle, the trig functions and capital letters go on top.