Primitive of Reciprocal of a x squared plus b x plus c/Examples/x^2 + 4 x + 5
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Example of Use of Primitive of $\dfrac 1 {a x^2 + b x + c}$
- $\ds \int \dfrac {\d x} {x^2 + 4 x + 5} = \map \arctan {x + 2} + C$
Proof
We aim to use Primitive of $\dfrac 1 {a x^2 + b x + c}$ with:
\(\ds a\) | \(=\) | \(\ds 1\) | ||||||||||||
\(\ds b\) | \(=\) | \(\ds 4\) | ||||||||||||
\(\ds c\) | \(=\) | \(\ds 5\) |
We note that:
\(\ds b^2 - 4 a c\) | \(=\) | \(\ds 4^2 - 4 \times 1 \times 5\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 16 - 20\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds -4\) |
Hence from Primitive of $\dfrac 1 {a x^2 + b x + c}$:
- $\ds \int \frac {\d x} {a x^2 + b x + c} = \dfrac 2 {\sqrt {4 a c - b^2} } \map \arctan {\dfrac {2 a x + b} {\sqrt {4 a c - b^2} } } + C$
Substituting for $a$, $b$ and $c$ and simplifying:
\(\ds \int \frac {\d x} {x^2 + 4 x + 5}\) | \(=\) | \(\ds \dfrac 2 {\sqrt {4 \times 1 \times 5 - 4^2} } \map \arctan {\dfrac {2 \times 1 \cdot x + 4} {\sqrt {4 \times 1 \times 5 - 4^2} } } + C\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 2 {\sqrt 4} \map \arctan {\dfrac {2 x + 4} {\sqrt 4} } + C\) | simplifying | |||||||||||
\(\ds \) | \(=\) | \(\ds \map \arctan {x + 2} + C\) | simplifying |
$\blacksquare$
Sources
- 1953: L. Harwood Clarke: A Note Book in Pure Mathematics ... (previous) ... (next): $\text {II}$. Calculus: Exercises $\text {XIV}$: $15$.