We need to find the mass $m$ and center of mass $(\bar{x}, \bar{y})$ of the lamina bounded by the curves $y = e^x$, $y = 0$, $x = 0$, $x = 1$, with density function $\delta(x, y) = 2 - x + y$.

AnalysisCalculusDouble IntegralsCenter of MassIntegration by PartsLamina
2025/5/25

1. Problem Description

We need to find the mass mm and center of mass (xˉ,yˉ)(\bar{x}, \bar{y}) of the lamina bounded by the curves y=exy = e^x, y=0y = 0, x=0x = 0, x=1x = 1, with density function δ(x,y)=2x+y\delta(x, y) = 2 - x + y.

2. Solution Steps

First, we calculate the mass mm of the lamina:
m=Rδ(x,y)dA=010ex(2x+y)dydxm = \int\int_R \delta(x, y) \, dA = \int_0^1 \int_0^{e^x} (2 - x + y) \, dy \, dx
We first evaluate the inner integral with respect to yy:
0ex(2x+y)dy=[(2x)y+12y2]0ex=(2x)ex+12e2x \int_0^{e^x} (2 - x + y) \, dy = \left[(2 - x)y + \frac{1}{2}y^2\right]_0^{e^x} = (2 - x)e^x + \frac{1}{2}e^{2x}
Now, we evaluate the outer integral with respect to xx:
m=01[(2x)ex+12e2x]dx=01(2x)exdx+1201e2xdx m = \int_0^1 \left[(2 - x)e^x + \frac{1}{2}e^{2x}\right] \, dx = \int_0^1 (2 - x)e^x \, dx + \frac{1}{2} \int_0^1 e^{2x} \, dx
We evaluate 01(2x)exdx\int_0^1 (2 - x)e^x \, dx using integration by parts: Let u=2xu = 2 - x, dv=exdxdv = e^x \, dx, so du=dxdu = -dx, v=exv = e^x. Then,
01(2x)exdx=[(2x)ex]0101ex(dx)=(21)e1(20)e0+01exdx=e2+[ex]01=e2+e1=2e3 \int_0^1 (2 - x)e^x \, dx = \left[(2 - x)e^x\right]_0^1 - \int_0^1 e^x (-dx) = (2 - 1)e^1 - (2 - 0)e^0 + \int_0^1 e^x \, dx = e - 2 + [e^x]_0^1 = e - 2 + e - 1 = 2e - 3
We evaluate 1201e2xdx\frac{1}{2} \int_0^1 e^{2x} \, dx:
1201e2xdx=12[12e2x]01=14(e21) \frac{1}{2} \int_0^1 e^{2x} \, dx = \frac{1}{2} \left[\frac{1}{2}e^{2x}\right]_0^1 = \frac{1}{4}(e^2 - 1)
Therefore,
m=2e3+14(e21)=2e3+14e214=14e2+2e134 m = 2e - 3 + \frac{1}{4}(e^2 - 1) = 2e - 3 + \frac{1}{4}e^2 - \frac{1}{4} = \frac{1}{4}e^2 + 2e - \frac{13}{4}
Next, we calculate MyM_y and MxM_x:
My=Rxδ(x,y)dA=010exx(2x+y)dydx M_y = \int\int_R x\delta(x, y) \, dA = \int_0^1 \int_0^{e^x} x(2 - x + y) \, dy \, dx
0exx(2x+y)dy=x[(2x)y+12y2]0ex=x(2x)ex+12xe2x=(2xx2)ex+12xe2x \int_0^{e^x} x(2 - x + y) \, dy = x\left[(2 - x)y + \frac{1}{2}y^2\right]_0^{e^x} = x(2 - x)e^x + \frac{1}{2}xe^{2x} = (2x - x^2)e^x + \frac{1}{2}xe^{2x}
My=01[(2xx2)ex+12xe2x]dx=01(2xx2)exdx+1201xe2xdx M_y = \int_0^1 \left[(2x - x^2)e^x + \frac{1}{2}xe^{2x}\right] \, dx = \int_0^1 (2x - x^2)e^x \, dx + \frac{1}{2} \int_0^1 xe^{2x} \, dx
We evaluate 01(2xx2)exdx\int_0^1 (2x - x^2)e^x \, dx using integration by parts twice:
x2exdx=x2ex2xexdx=x2ex2(xexex)\int x^2 e^x dx = x^2 e^x - \int 2x e^x dx = x^2 e^x - 2(xe^x - e^x)
2xexdx=2xexdx=2(xexex)\int 2x e^x dx = 2 \int xe^x dx = 2(xe^x - e^x)
So (2xx2)exdx=2(xexex)(x2ex2xex+2ex)=(4xx2)ex4ex\int (2x-x^2)e^x dx = 2(xe^x - e^x) - (x^2 e^x - 2xe^x + 2e^x) = (4x-x^2)e^x-4e^x
01(2xx2)exdx=(41)e4e(04)=e+4\int_0^1 (2x-x^2)e^x dx = (4-1)e-4e - (0-4) = -e + 4
We evaluate 1201xe2xdx\frac{1}{2}\int_0^1 x e^{2x}dx using integration by parts:
u=xu=x, dv=e2xdxdv = e^{2x} dx, so du=dxdu=dx v=12e2xv = \frac{1}{2} e^{2x}
Then, 1201xe2xdx=12[x(12e2x)010112e2xdx]=12[12e212(12e2x)01]=14e214(12e212)=14e218e2+18=18e2+18\frac{1}{2} \int_0^1 xe^{2x} dx = \frac{1}{2}[x (\frac{1}{2}e^{2x})|_0^1 - \int_0^1 \frac{1}{2}e^{2x} dx] = \frac{1}{2}[ \frac{1}{2}e^2 - \frac{1}{2} (\frac{1}{2}e^{2x})|_0^1] = \frac{1}{4}e^2 - \frac{1}{4}(\frac{1}{2}e^2 - \frac{1}{2}) = \frac{1}{4}e^2 - \frac{1}{8}e^2 + \frac{1}{8} = \frac{1}{8}e^2 + \frac{1}{8}
My=e+4+18e2+18=18e2e+338M_y = -e+4 + \frac{1}{8}e^2 + \frac{1}{8} = \frac{1}{8}e^2 - e + \frac{33}{8}
xˉ=Mym=18e2e+33814e2+2e134=e28e+332e2+16e26\bar{x} = \frac{M_y}{m} = \frac{\frac{1}{8}e^2 - e + \frac{33}{8}}{\frac{1}{4}e^2 + 2e - \frac{13}{4}} = \frac{e^2 - 8e + 33}{2e^2 + 16e - 26}
Mx=Ryδ(x,y)dA=010exy(2x+y)dydx M_x = \int\int_R y\delta(x, y) \, dA = \int_0^1 \int_0^{e^x} y(2 - x + y) \, dy \, dx
0exy(2x+y)dy=0ex(2yxy+y2)dy=[y212xy2+13y3]0ex=e2x12xe2x+13e3x \int_0^{e^x} y(2 - x + y) \, dy = \int_0^{e^x} (2y - xy + y^2) \, dy = \left[y^2 - \frac{1}{2}xy^2 + \frac{1}{3}y^3\right]_0^{e^x} = e^{2x} - \frac{1}{2}xe^{2x} + \frac{1}{3}e^{3x}
Mx=01[e2x12xe2x+13e3x]dx=01e2xdx1201xe2xdx+1301e3xdx M_x = \int_0^1 \left[e^{2x} - \frac{1}{2}xe^{2x} + \frac{1}{3}e^{3x}\right] \, dx = \int_0^1 e^{2x} dx - \frac{1}{2}\int_0^1 xe^{2x} dx + \frac{1}{3}\int_0^1 e^{3x} dx
We know that 01e2xdx=12e212\int_0^1 e^{2x} dx = \frac{1}{2}e^2 - \frac{1}{2} and 01xe2xdx=14e2+14\int_0^1 xe^{2x} dx = \frac{1}{4}e^2 + \frac{1}{4}, and 01e3xdx=13e313\int_0^1 e^{3x}dx = \frac{1}{3} e^3 - \frac{1}{3}.
So Mx=(12e212)12(14e2+14)+13(13e313)=12e21218e218+19e319=19e3+38e24972M_x = (\frac{1}{2}e^2 - \frac{1}{2}) - \frac{1}{2} (\frac{1}{4}e^2 + \frac{1}{4}) + \frac{1}{3}(\frac{1}{3}e^3 - \frac{1}{3}) = \frac{1}{2}e^2 - \frac{1}{2} - \frac{1}{8}e^2 - \frac{1}{8} + \frac{1}{9}e^3 - \frac{1}{9} = \frac{1}{9}e^3 + \frac{3}{8}e^2 - \frac{49}{72}
yˉ=Mxm=19e3+38e2497214e2+2e134=8e3+27e24918e2+144e117\bar{y} = \frac{M_x}{m} = \frac{\frac{1}{9}e^3 + \frac{3}{8}e^2 - \frac{49}{72}}{\frac{1}{4}e^2 + 2e - \frac{13}{4}} = \frac{8e^3 + 27e^2 - 49}{18e^2 + 144e - 117}

3. Final Answer

m=14e2+2e134m = \frac{1}{4}e^2 + 2e - \frac{13}{4}
xˉ=e28e+332e2+16e26\bar{x} = \frac{e^2 - 8e + 33}{2e^2 + 16e - 26}
yˉ=8e3+27e24918e2+144e117\bar{y} = \frac{8e^3 + 27e^2 - 49}{18e^2 + 144e - 117}

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