00:01
Sometimes with complicated integrals, it's the best to do what's called a u substitution.
00:06
U substitution is when you take just one part of your integral that you recognize is a derivative of another part, and then that can lead to some cancellations and help make your integral a lot easier.
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So let's look at some examples of u substitutions.
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Let's say we're trying to integrate the integral of sinex over square root of 1 plus cosine x, dx.
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Now that square root part is kind of ugly.
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It's always hard to integrate when there's a square root involved.
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But if i look under the square root, i know that cosine and sign are connected by differentiation because i know that the derivative of cosine is negative sign.
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So i'm going to take everything under the square root and i'm going to do a u substitution with that 1 plus cosine x.
01:01
So then i'm going to differentiate my u.
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The liturative is of any constant.
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It's just zero.
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The derivative of cosine is negative sine x dx.
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And then i basically want to solve this for dx, because i'm going to replace dx with negative 1 over sine x du.
01:23
So i just divided both sides by like negative sine x to get dx on its own.
01:28
So now i can rewrite this, right? i can replace, remember, everything on this square root now was just a u.
01:36
And now my dx is negative 1 over sine x, du.
01:44
So, like, this is important because now my, the sine x is cancel out.
01:50
And this negative sign, because basically a constant, can be moved outside of the integral.
01:57
So now i have negative 1 over square root of u, du.
02:03
And it usually helps here to rewrite our like square root in terms of an exponent raised to the one half.
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And because it's in the denominator is actually going to be raised to the negative one half.
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So we have u to the negative one half du.
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And then anytime that we're integrating these, we can use the reverse power rule where we add one to the exponent.
02:32
So negative 1⁄2 plus 1 is positive 1 half, and then we divide by that new exponent.
02:43
So this would be our solution so far.
02:47
But when we're dividing by a fraction, it's the same thing as multiplying by the reciprocal.
02:53
Right.
02:54
The reciprocal of 1⁄2 is 2.
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So we have negative 2u to the 1⁄2 plus c.
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And our final step is to now undo our use of 1⁄2.
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So remember at the beginning, we said u is one plus cosine x.
03:11
So now we replace you again with one plus cosine x to the one half plus c.
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And then if we want, we can turn our one half again into a square root.
03:26
One plus cosine x plus c.
03:28
That's kind of like an optional step...