Conclusion: Option C is correct.

You can compare it with this question: 1987 Chinese 16.

Know the value of.

Solution 1

Solution 2

Set to the angle of the fourth quadrant, if, then

solve

It's also VIII

∴

∫ is the fourth quadrant angle,

∴ ,

∴ ,

∴ ,

,

Refine and improve

There are four formulas for sum, difference and product * * *:

In the process of solving the first three problems, the sum-difference product formula is used.

Elementary mathematics is a very mature content, but different teachers will have different views on teaching methods.

For trigonometric functions, some teachers will advise students to memorize more formulas, such as the triple angle formula. In my opinion, the triple angle formula is far less important and less used than the sum-difference product formula. This kind of formula, which is not used much, is easily mistaken and confused. If you use the wrong formula in the exam and lose points, it will be a big loss.

In the final analysis, learning mathematics is learning deduction; You can't learn math by rote.

In fact, the triple angle formula can be easily derived by using the sum-difference product formula.

∵

∴

∵

∴