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Trig question

How did they get from line 1 to 2?

Using the rule for cos (a+b):
sina sinb = cos (a+b) - cosa cosb

How did they end up with only two trig functions and not three.
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Reply 1

TeeEm

Original post by cooldudeman

How did they get from line 1 to 2?

Using the rule for cos (a+b):
sina sinb = cos (a+b) - cosa cosb

How did they end up with only two trig functions and not three.
Posted from TSR Mobile

shouldn't that be minus?

Reply 2

davros

Original post by cooldudeman

How did they get from line 1 to 2?

Using the rule for cos (a+b):
sina sinb = cos (a+b) - cosa cosb

How did they end up with only two trig functions and not three.
Posted from TSR Mobile

If you expand both cos functions you get four terms, but two of them cancel out and the other two are the same, so you are left with the product of two sines.

Alternatively, you should recognize this as one of the factor formulae from A level - your formula book would have contained a formula for sinAsinB etc.