We need to evaluate the definite integral: $\int_{-1}^{0} \frac{dx}{4x^2 + 8x + 8}$

AnalysisDefinite IntegralIntegrationSubstitutionCompleting the SquareTrigonometric Functions
2025/5/19

1. Problem Description

We need to evaluate the definite integral:
10dx4x2+8x+8\int_{-1}^{0} \frac{dx}{4x^2 + 8x + 8}

2. Solution Steps

First, factor out the constant 4 from the denominator:
10dx4x2+8x+8=10dx4(x2+2x+2)\int_{-1}^{0} \frac{dx}{4x^2 + 8x + 8} = \int_{-1}^{0} \frac{dx}{4(x^2 + 2x + 2)}
Then,
10dx4(x2+2x+2)=1410dxx2+2x+2\int_{-1}^{0} \frac{dx}{4(x^2 + 2x + 2)} = \frac{1}{4} \int_{-1}^{0} \frac{dx}{x^2 + 2x + 2}
Complete the square for the expression in the denominator:
x2+2x+2=(x2+2x+1)+1=(x+1)2+1x^2 + 2x + 2 = (x^2 + 2x + 1) + 1 = (x+1)^2 + 1
Substitute this back into the integral:
1410dx(x+1)2+1\frac{1}{4} \int_{-1}^{0} \frac{dx}{(x+1)^2 + 1}
Use the substitution u=x+1u = x+1, so du=dxdu = dx. When x=1x=-1, u=1+1=0u = -1+1=0. When x=0x=0, u=0+1=1u = 0+1=1.
1401duu2+1\frac{1}{4} \int_{0}^{1} \frac{du}{u^2 + 1}
The integral of 1u2+1\frac{1}{u^2 + 1} is arctan(u)\arctan(u). Thus, we have:
14[arctan(u)]01=14(arctan(1)arctan(0))\frac{1}{4} [\arctan(u)]_{0}^{1} = \frac{1}{4} (\arctan(1) - \arctan(0))
Since arctan(1)=π4\arctan(1) = \frac{\pi}{4} and arctan(0)=0\arctan(0) = 0, we have:
14(π40)=π16\frac{1}{4} (\frac{\pi}{4} - 0) = \frac{\pi}{16}

3. Final Answer

π16\frac{\pi}{16}

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