why cant i use substitution for this?

Why doesn't substitution work to find 0∫2 sqrt(4-x^2). I kn ow you can find the integral in other ways. I am just curious why regular substitution won't work. Thank you in advance.
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 Quote by schapman22 Why doesn't substitution work to find 0∫2 sqrt(4-x^2). I kn ow you can find the integral in other ways. I am just curious why regular substitution won't work. Thank you in advance.
If you make the substitution $u=x^2$ then du = 2x dx. But you don't have a factor of 2x in the integrand. Or were you thinking of a different substitution? In any case, a trigonometric substitution would work better.

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 Quote by schapman22 Why doesn't substitution work to find 0∫2 sqrt(4-x^2). I kn ow you can find the integral in other ways. I am just curious why regular substitution won't work. Thank you in advance.
But substitution does work. Make the substitution x = 2cos(θ). Isn't that a "substitution"?