Formulas that can be used directly are all in textbooks, no matter examples or exercises, there is nothing else. And the examiner's idea is different. Even if there is a standard answer, everyone has a different understanding of this formula. If the final conclusion is completely correct, it is estimated that the examiner will not care too much about your use. Once the result is wrong, it is easy to deduct points. The ideal way not to be stranded is that you can follow the standard answer. Using this formula is always risky unless you can prove the formula itself ... but if you have time to prove it, you can use other methods.