It is important that you memorize the below three addition formulas as it is necessary to establish your foundations in trigonometry. From these addition formulas, you will be able to derive the double angle formula.
Let A and B be any two angles. Then
Without the use of tables or calculator, find the value of
We have to figure out the common angles like
that add up to
. We do this because we know the numerical values of cosine and sine of these common angles. Then we expand it out using addition formulas and solve for theanswer.
This is similar as well. It's just that in this example, subtraction is required to obtain
. Now, we are using the addition formula for tangent.
If we had used the calculator instead of addition formulas , it would have been very difficult to arrive at the exact value that was required. No credit would have been rewarded.
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