# What's the common theme to look out for, while applying Feynman's Integral tricks? [duplicate]

I have seen Feynman's integral trick coming into use in many questions, but I don't really see a common way to recognize the format for it. Of course when a question comes with an explicit parameter, it's easy to recognize the pattern and follow the steps known to us in the form of Feynman's Trick. So, is there any standard way to quickly recognize the same being in action for any asked question, i.e., how to observe that the question intends us to make use of Feynman's method?

• I did try to find a relevant thread for the same, but I couldn't come across anything that would give the clarity needed. But, in case I missed a few threads- I will also appreciate if I could be linked to those threads. Aug 1, 2022 at 2:19
• For 90% of the encountered cases it's useless. It is usable only in some very few niche questions Aug 1, 2022 at 2:21
• Yeah, this helps a lot! Are there more threads like these and, if yes, could you link me to them as well? Aug 1, 2022 at 2:28
• I liked this medium article cantorsparadise.com/… Aug 1, 2022 at 2:32
• Thanks, I have gone through this and it's a good read! Aug 1, 2022 at 2:34

One trick I found out is that if you see an integral of the form $$\int_a^b \frac{f(h(x))}{h(x)g(x)}$$, you can let $$I(a) = \int_a^b \frac{f(ah(x))}{h(x)g(x)}$$ then take the partial derivative with respect to $$a$$ to get $$I'(a) = \int_a^b \frac{f'(ah(x))}{g(x)}$$, which could be easier to work with depending on what $$f$$ and $$g$$ are.