Why cant i use substitution for this?

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SUMMARY

The discussion centers on the challenges of using substitution to evaluate the integral 0∫2 sqrt(4-x^2). While a direct substitution such as u=x^2 fails due to the absence of the necessary factor (2x), a trigonometric substitution, specifically x = 2cos(θ), is effective. Participants clarify that substitution can work, but the choice of substitution is critical for successful integration. The conversation emphasizes the importance of selecting appropriate substitutions in integral calculus.

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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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schapman22 said:
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 [itex]u=x^2[/itex] 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.
 
schapman22 said:
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"?
 

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