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kakasveta [241]
4 years ago
9

Integral of 1/sqrt (81-225x^2)

Mathematics
1 answer:
ehidna [41]4 years ago
8 0
\displaystyle\int\frac{\mathrm dx}{\sqrt{81-225x^2}}

Let x=\dfrac9{15}\sin t, so that \mathrm dx=\dfrac9{15}\cos t\,\mathrm dt and the integral becomes

\displaystyle\int\frac{\dfrac9{15}\cos t}{\sqrt{81-225\left(\frac9{15}\sin t\right)^2}}\,\mathrm dt
\displaystyle\int\frac{\dfrac9{15}\cos t}{\sqrt{81-225\times\frac{81}{225}\sin^2 t}}\,\mathrm dt
\displaystyle\frac1{15}\int\frac{\cos t}{\sqrt{1-\sin^2 t}}\,\mathrm dt
\displaystyle\frac1{15}\int\frac{\cos t}{\sqrt{\cos^2 t}}\,\mathrm dt
\displaystyle\frac1{15}\int\frac{\cos t}{|\cos t|}\,\mathrm dt

When \cos t>0, you have |\cos t|=\cos t. Under these conditions, you can write

\displaystyle\frac1{15}\int\frac{\cos t}{\cos t}\,\mathrm dt=\frac1{15}\int\mathrm dt=\frac t{15}+C

and back-substituting yields

\dfrac{\arcsin\frac{15x}9}{15}+C=\dfrac{\arcsin\frac{5x}3}{15}+C
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