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→Trigonometric Substitution
===Trigonometric Substitution===
===Trigonometric Substitution===
'''Integration by Trigonometric Substitution''' is a [[Techniques of integration|technique]] to facilitate the integration of a rational expression by substituting a more complicated radical expression with a trigonometric expression.
Given an integral
:<big><math>\int\frac{1}{\sqrt{9-x^2}}dx</math></big>
By looking at the radical we can determine that it represents the base of a right triangle by understanding the [[Pythagorean theorem]].
:<big><math>\sqrt{9-x^2}=3+x</math></big> where 3 is the hypotenuse and x is the height of the triangle.
This allows us to rewrite the expression to :<big><math>sin\theta=\frac{x}{3}</math></big>. This allows us to substitute x with :<big><math>3sin\theta</math></big>.
Now to do the substitution
:<big><math>9-x^2=9-(3sin\theta )^2</math></big>
:<big><math>9-(3sin\theta )^2=9-9sin^2\theta </math></big>
:<big><math>9-9sin^2\theta=9(1- sin^2\theta) </math></big>
And by use of trigonometric identities we know that
:<big><math>1- sin^2\theta=cos^2\theta </math></big>
:<big><math>9(1- sin^2\theta)=9cos^2\theta </math></big>
Therefore
:<big><math>\sqrt{9-x^2}=3cos\theta </math></big>
We are not done yet, we must also take the derivative of :<big><math>3sin\theta</math></big>
:<big><math>\frac{dx}{d\theta}3sin\theta=3cos\theta </math></big>
By partial derivatives we move the :<big><math>{d\theta}</math></big> over.
:<big><math>dx=3cos\theta d\theta </math></big>
Now we are ready to rewrite our integral.
:<big><math>\int\frac{1}{\sqrt{9-x^2}}dx=\int\frac{3cos\theta}{3cos\theta}d\theta=\int d\theta=\theta+c</math></big>
From our trigonometric expression :<big><math>x=3sin\theta</math></big> we can see that
:<big><math>\theta=sin^{-1}(\frac{x}{3})+c</math></big> giving us the final solution.
:<big><math>\int\frac{1}{\sqrt{9-x^2}}dx=sin^{-1}(\frac{x}{3})+c </math></big>