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Find the center of mass of the region bounded by the following functions.


y=ln x,quad x=1,quad x=2,quad y=0

99. Find the center of mass of the region bounded by the following functions.\newliney=lnx,x=1,x=2,y=0 y=\ln x, \quad x=1, \quad x=2, \quad y=0

Full solution

Q. 99. Find the center of mass of the region bounded by the following functions.\newliney=lnx,x=1,x=2,y=0 y=\ln x, \quad x=1, \quad x=2, \quad y=0
  1. Calculate xˉ\bar{x}: To find the center of mass, we need to calculate the coordinates (xˉ\bar{x}, yˉ\bar{y}). First, let's find xˉ\bar{x}, which is the average x-value weighted by area.
  2. Find xˉ\bar{x} formula: The formula for xˉ\bar{x} is the integral of xx times the density function over the area, divided by the total mass (area). Since density is constant, it cancels out. So, xˉ=(1/Area)×abx(y)dy\bar{x} = (1/\text{Area}) \times \int_{a}^{b} x(y) \, dy, where y=lnxy=\ln x and the bounds are x=1x=1 and x=2x=2.
  3. Calculate area: We need to find the area first. The area is given by the integral from 11 to 22 of lnxdx\ln x \, dx.
  4. Calculate Area: Calculating the integral, we get Area = 12lnxdx=[xlnxx]12\int_{1}^{2} \ln x \, dx = [x \ln x - x]_{1}^{2}.
  5. Calculate xˉ\bar{x}: Plugging in the bounds, Area = (2ln22)(1ln11)=2ln22(01)=2ln21(2 \ln 2 - 2) - (1 \ln 1 - 1) = 2 \ln 2 - 2 - (0 - 1) = 2 \ln 2 - 1.
  6. Calculate integral: Now, we calculate xˉ=1Area12xlnxdx\bar{x} = \frac{1}{\text{Area}} \int_{1}^{2} x \ln x \, dx.
  7. Evaluate integral: The integral of xlnxdxx \ln x \, dx is x22lnxx24\frac{x^2}{2} \ln x - \frac{x^2}{4}. So, we need to evaluate this from 11 to 22.
  8. Calculate xˉ\bar{x}: Plugging in the bounds, we get [222ln2224][122ln1124]=[2ln21][014]=2ln21+14[\frac{2^2}{2} \ln 2 - \frac{2^2}{4}] - [\frac{1^2}{2} \ln 1 - \frac{1^2}{4}] = [2 \ln 2 - 1] - [0 - \frac{1}{4}] = 2 \ln 2 - 1 + \frac{1}{4}.
  9. Simplify xˉ\bar{x}: Now, we plug the area and the integral result into the formula for xˉ\bar{x}. xˉ=(12ln21)(2ln21+14)\bar{x} = \left(\frac{1}{2 \ln 2 - 1}\right) \cdot \left(2 \ln 2 - 1 + \frac{1}{4}\right).
  10. Identify mistake: Simplifying, we get xˉ=2ln21+142ln21\bar{x} = \frac{2 \ln 2 - 1 + \frac{1}{4}}{2 \ln 2 - 1}. Wait, there's a mistake here. We didn't multiply xx by lnx\ln x in the integral for xˉ\bar{x}.

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