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Evaluate the integral and express your answer in simplest form.

int(3)/(xsqrt(x^(2)-36))dx
Answer:

Evaluate the integral and express your answer in simplest form.\newline3xx236dx \int \frac{3}{x \sqrt{x^{2}-36}} d x \newlineAnswer:

Full solution

Q. Evaluate the integral and express your answer in simplest form.\newline3xx236dx \int \frac{3}{x \sqrt{x^{2}-36}} d x \newlineAnswer:
  1. Recognize integral form: Let's start by recognizing that the integral resembles the form of the derivative of an inverse trigonometric function, specifically the inverse hyperbolic function. We can use a substitution to simplify the integral. Let's set x=6sec(θ)x = 6\sec(\theta), which implies dx=6sec(θ)tan(θ)dθdx = 6\sec(\theta)\tan(\theta)d\theta. When x=6x = 6, θ=0\theta = 0, and as xx approaches infinity, θ\theta approaches π2\frac{\pi}{2}. The integral becomes:\newline3xx236dx=36sec(θ)36sec2(θ)366sec(θ)tan(θ)dθ\int\frac{3}{x\sqrt{x^2-36}}dx = \int\frac{3}{6\sec(\theta)\sqrt{36\sec^2(\theta)-36}} \cdot 6\sec(\theta)\tan(\theta)d\theta
  2. Simplify with substitution: Simplify the integral by substituting x=6sec(θ)x = 6\sec(\theta) and dx=6sec(θ)tan(θ)dθdx = 6\sec(\theta)\tan(\theta)d\theta:\newline36sec(θ)36sec2(θ)366sec(θ)tan(θ)dθ\int\frac{3}{6\sec(\theta)\sqrt{36\sec^2(\theta)-36}} \cdot 6\sec(\theta)\tan(\theta)d\theta\newline= 36sec(θ)36(sec2(θ)1)6sec(θ)tan(θ)dθ\int\frac{3}{6\sec(\theta)\sqrt{36(\sec^2(\theta)-1)}} \cdot 6\sec(\theta)\tan(\theta)d\theta\newline= 36sec(θ)36tan2(θ)6sec(θ)tan(θ)dθ\int\frac{3}{6\sec(\theta)\sqrt{36\tan^2(\theta)}} \cdot 6\sec(\theta)\tan(\theta)d\theta\newline= 3636tan(θ)6sec(θ)tan(θ)dθ\int\frac{3}{6\sqrt{36}\tan(\theta)} \cdot 6\sec(\theta)\tan(\theta)d\theta\newline= 366tan(θ)6sec(θ)tan(θ)dθ\int\frac{3}{6\cdot 6\tan(\theta)} \cdot 6\sec(\theta)\tan(\theta)d\theta\newline= 336tan(θ)6sec(θ)tan(θ)dθ\int\frac{3}{36\tan(\theta)} \cdot 6\sec(\theta)\tan(\theta)d\theta\newline= 16sec(θ)dθ\int\frac{1}{6}\sec(\theta)d\theta
  3. Integrate secant function: Now we integrate (16)sec(θ)dθ(\frac{1}{6})\sec(\theta)d\theta:\newline\int(\frac{\(1\)}{\(6\)})\sec(\theta)d\theta = (\frac{\(1\)}{\(6\)})\ln|\sec(\theta) + \tan(\theta)| + C
  4. Back-substitute for x: We need to back-substitute to return to the original variable x. From our substitution, we have \(x = 6\sec(\theta), so sec(θ)=x6\sec(\theta) = \frac{x}{6}. Also, tan(θ)=x2366\tan(\theta) = \frac{\sqrt{x^2-36}}{6}. Therefore, the integral in terms of x is:\newline\frac{\(1\)}{\(6\)}\ln|\sec(\theta) + \tan(\theta)| + C\(\newline= \frac{11}{66}\ln|\left(\frac{x}{66}\right) + \left(\frac{\sqrt{x^2236-36}}{66}\right)| + C\newline= \frac{11}{66}\ln|\frac{x + \sqrt{x^2236-36}}{66}| + C\newline= \frac{11}{66}\ln|x + \sqrt{x^2236-36}| - \frac{11}{66}\ln(66) + C
  5. Combine constant terms: We can combine the constant terms into a single constant of integration:\newline(\frac{1}{6})\ln|x + \sqrt{(x^2-36)}| - (\frac{1}{6})\ln(6) + C\(\newline= (\frac{1}{6})\ln|x + \sqrt{(x^2-36)}| - \ln(6^{(\frac{1}{6})}) + C\newline= (\frac{1}{6})\ln|x + \sqrt{(x^2-36)}| + C'\)\newlinewhere CC' is a new constant of integration that absorbs the ln(6(16))-\ln(6^{(\frac{1}{6})}) term.