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Evaluate the integral and express your answer in simplest form.

int(3)/(xsqrt(x^(2)-9))dx
Answer:

Evaluate the integral and express your answer in simplest form.\newline3xx29dx \int \frac{3}{x \sqrt{x^{2}-9}} d x \newlineAnswer:

Full solution

Q. Evaluate the integral and express your answer in simplest form.\newline3xx29dx \int \frac{3}{x \sqrt{x^{2}-9}} d x \newlineAnswer:
  1. Recognize standard inverse trigonometric form: Recognize the integral as a standard inverse trigonometric form.\newlineThe integral resembles the form of the derivative of the inverse hyperbolic function, specifically the inverse hyperbolic secant. We can rewrite the integral in a form that matches the derivative of arcsinh(x/a)\text{arcsinh}(x/a) or arcsech(x/a)\text{arcsech}(x/a).\newlineI=3xx29dxI = \int \frac{3}{x\sqrt{x^2-9}}dx
  2. Use substitution to simplify: Use substitution to simplify the integral. Let x=3sec(θ)x = 3\sec(\theta), which implies dx=3sec(θ)tan(θ)dθdx = 3\sec(\theta)\tan(\theta)d\theta. When x=3sec(θ)x = 3\sec(\theta), we have x29=9sec2(θ)9=3tan(θ)\sqrt{x^2 - 9} = \sqrt{9\sec^2(\theta) - 9} = 3\tan(\theta). Now substitute these into the integral. I=33sec(θ)3tan(θ)3sec(θ)tan(θ)dθI = \int\frac{3}{3\sec(\theta)*3\tan(\theta)} * 3\sec(\theta)\tan(\theta)d\theta
  3. Simplify the expression: Simplify the expression.\newlineThe 3sec(θ)tan(θ)3\sec(\theta)\tan(\theta) terms cancel out, leaving us with:\newlineI=1tan(θ)dθI = \int\frac{1}{\tan(\theta)}d\theta
  4. Recognize natural logarithm function: Recognize the integral as the natural logarithm function.\newlineThe integral of 1tan(θ)\frac{1}{\tan(\theta)} is the natural logarithm of the absolute value of sin(θ)\sin(\theta).\newlineI=lnsin(θ)+CI = \ln|\sin(\theta)| + C, where CC is the constant of integration.
  5. Back-substitute to original variable: Back-substitute to return to the original variable xx. We need to express sin(θ)\sin(\theta) in terms of xx. From the substitution x=3sec(θ)x = 3\sec(\theta), we have cos(θ)=3x\cos(\theta) = \frac{3}{x}. Since sin2(θ)+cos2(θ)=1\sin^2(\theta) + \cos^2(\theta) = 1, we can solve for sin(θ)=1cos2(θ)=1(3x)2\sin(\theta) = \sqrt{1 - \cos^2(\theta)} = \sqrt{1 - \left(\frac{3}{x}\right)^2}. I=ln1(3x)2+CI = \ln|\sqrt{1 - \left(\frac{3}{x}\right)^2}| + C
  6. Simplify expression inside logarithm: Simplify the expression inside the logarithm.\newlineWe can simplify the square root and the square inside the logarithm.\newlineI=ln19/x2+CI = \ln|\sqrt{1 - 9/x^2}| + C\newlineI=ln1xx29+CI = \ln|\frac{1}{x} \cdot \sqrt{x^2 - 9}| + C
  7. Simplify logarithm expression: Simplify the logarithm expression.\newlineSince ln(ab)=ln(a)+ln(b)\ln(a\cdot b) = \ln(a) + \ln(b), we can separate the terms inside the logarithm.\newlineI=ln1x+lnx29+CI = \ln\left|\frac{1}{x}\right| + \ln\left|\sqrt{x^2 - 9}\right| + C
  8. Combine constants into single constant: Simplify further using properties of logarithms.\newlineThe natural logarithm of a square root is one-half the natural logarithm of the argument, and ln1x\ln|\frac{1}{x}| is lnx-\ln|x|.\newlineI=lnx+12lnx29+CI = -\ln|x| + \frac{1}{2} \cdot \ln|x^2 - 9| + C
  9. Combine constants into single constant: Simplify further using properties of logarithms.\newlineThe natural logarithm of a square root is one-half the natural logarithm of the argument, and ln1x\ln|\frac{1}{x}| is lnx-\ln|x|.\newlineI=lnx+12lnx29+CI = -\ln|x| + \frac{1}{2} \cdot \ln|x^2 - 9| + CCombine the constants into a single constant of integration.\newlineSince CC is an arbitrary constant, we can combine the constants from the logarithm properties into a single constant.\newlineI=lnx+12lnx29+CI = -\ln|x| + \frac{1}{2} \cdot \ln|x^2 - 9| + C