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Evaluate the integral: int(2)/(xsqrt(x^(2)-36))dx

Evaluate the integral: 2xx236dx \int \frac{2}{x \sqrt{x^{2}-36}} d x

Full solution

Q. Evaluate the integral: 2xx236dx \int \frac{2}{x \sqrt{x^{2}-36}} d x
  1. Identify Integral: Let's identify the integral we need to solve:\newlineI=2xx236dxI = \int \frac{2}{x\sqrt{x^2-36}}dx\newlineWe can see that this integral suggests a trigonometric substitution because of the form a2x2a^2 - x^2 under the square root. We will use the substitution x=asec(θ)x = a\sec(\theta), where a=6a = 6 in this case, because 36=6236 = 6^2.
  2. Substitute xx: Substitute x=6sec(θ)x = 6\sec(\theta) into the integral. Then, dx=6sec(θ)tan(θ)dθdx = 6\sec(\theta)\tan(\theta)d\theta and x236=36sec2(θ)36=36(tan2(θ))x^2 - 36 = 36\sec^2(\theta) - 36 = 36(\tan^2(\theta)). The integral becomes:\newlineI=26sec(θ)36tan2(θ)6sec(θ)tan(θ)dθI = \int\frac{2}{6\sec(\theta)\sqrt{36\tan^2(\theta)}} \cdot 6\sec(\theta)\tan(\theta)d\theta
  3. Simplify Integral: Simplify the integral by canceling terms and using the identity tan2(θ)=tan(θ)\sqrt{\tan^2(\theta)} = |\tan(\theta)|. Since we are dealing with x > 6 (because of the original square root), \sec(\theta) > 0, and thus \tan(\theta) > 0, we can remove the absolute value:\newlineI=26sec(θ)tan(θ)6sec(θ)tan(θ)dθI = \int\frac{2}{6\sec(\theta)\tan(\theta)} \cdot 6\sec(\theta)\tan(\theta)d\theta\newlineI=2dθI = \int 2d\theta
  4. Integrate with θ:\theta: Integrate with respect to θ:\theta:I=2θ+CI = 2\theta + C, where CC is the constant of integration.
  5. Substitute back for θ\theta: We need to substitute back for θ\theta using our original substitution x=6sec(θ)x = 6\sec(\theta). We have sec(θ)=x6\sec(\theta) = \frac{x}{6}, and to find θ\theta, we use the definition of secant: sec(θ)=1cos(θ)\sec(\theta) = \frac{1}{\cos(\theta)}, so cos(θ)=6x\cos(\theta) = \frac{6}{x}. Then θ=cos1(6x)\theta = \cos^{-1}(\frac{6}{x}).
    I=2cos1(6x)+CI = 2\cos^{-1}(\frac{6}{x}) + C