Primitive of x over Root of a squared minus x squared

Theorem

$\ds \int \frac {x \rd x} {\sqrt {a^2 - x^2} } = -\sqrt {a^2 - x^2} + C$

Proof

Let:

\(\ds z^2\)

\(=\)

\(\ds a^2 - x^2\)

\(\ds \leadsto \ \ \)

\(\ds 2 z \frac {\d z} {\d x}\)

\(=\)

\(\ds -2 x\)

Chain Rule for Derivatives, Power Rule for Derivatives

\(\ds \leadsto \ \ \)

\(\ds \int \frac {x \rd x} {\sqrt {a^2 - x^2} }\)

\(=\)

\(\ds \int -\frac {z \rd z} z\)

Integration by Substitution

\(\ds \)

\(=\)

\(\ds -\int \rd z\)

\(\ds \)

\(=\)

\(\ds -z + C\)

Primitive of Constant

\(\ds \)

\(=\)

\(\ds -\sqrt {a^2 - x^2} + C\)

substituting for $z$

$\blacksquare$

Also see

• Primitive of $\dfrac x {\sqrt {x^2 + a^2} }$
• Primitive of $\dfrac x {\sqrt {x^2 - a^2} }$

Sources

• 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 14$: Integrals involving $\sqrt {a^2 - x^2}$: $14.238$