Mammoth Memory

Where does the sine rule come from?

For the mathematical puritans out there the sine rule derives from the following:

(you really don’t need to know this)

Sine and Cosine are used to find the angle of a shape

Find “a” KNOWING the angle A and length b and c.

Redraw the diagram

Finding the length of A if B and C are known

Find Y in the triangle I 

First find the length of Y, the height of the triangle

Use trigonometry which tells us that `sin\ theta=(O)/H`


Therefore: `Y=bsinA`                                                    

Find “a” in triangle II

You can now find A in conjunction with sine

Use trigonometry which tells us that `sin\ theta=(O)/H`


Therefore  `a=(bsinA)/sinB`

Or  `a/sinA=b/sinB`  (the sine rule)

But again we emphasise that you really don’t need this.

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