What do you mean if it's possible? Technically, you can always use u-substitution. Most of the time, though, you don't need it because you should have memorized the integrals for basic questions, like the power rule and such.
Sometimes it is trial and error to figure out which piece of the integral to set equal to u. For this, there is no easy answer, sorry! I look for things inside parentheses with powers on them and try those first. Or in general, look over the problem and see if you can find the part that's making you use u-substitution in the first place.
Or, look for a derivative of a piece already present in the integral. For example, if you see an x^3, but then also see a 3x^2, you can probably use the piece containing x^3 as u because du will need to have 3x^2 in it, which you've already found.
Sometimes it is trial and error to figure out which piece of the integral to set equal to u. For this, there is no easy answer, sorry! I look for things inside parentheses with powers on them and try those first. Or in general, look over the problem and see if you can find the part that's making you use u-substitution in the first place.
Or, look for a derivative of a piece already present in the integral. For example, if you see an x^3, but then also see a 3x^2, you can probably use the piece containing x^3 as u because du will need to have 3x^2 in it, which you've already found.
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u - substitution comes in handy when you are integrating a problem with a high number of exponent or it consists of too many multiplication. Also if it's a square root of fraction.
Hope it helps out. :)
Hope it helps out. :)
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Sometimes it's obvious and oftentimes it's trial and error. Do a lot of practice problems.