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Exercise: Standard Forms — Square Roots · Q29

Q.Evaluate ∫x2+4x+13 dx\displaystyle\int \sqrt{x^2+4x+13}\,dx.

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Complete the square: x2+4x+13=(x+2)2+32x^2+4x+13=(x+2)^2+3^2. With u=x+2u=x+2, a=3a=3, the formula

∫u2+a2 du=u2u2+a2+a22ln⁡∣u+u2+a2∣+C\int\sqrt{u^2+a^2}\,du=\frac u2\sqrt{u^2+a^2}+\frac{a^2}2\ln\left|u+\sqrt{u^2+a^2}\right|+C (Section 6) gives

∫x2+4x+13 dx=x+22x2+4x+13+92ln⁡∣(x+2)+x2+4x+13∣+C.\int\sqrt{x^2+4x+13}\,dx = \frac{x+2}{2}\sqrt{x^2+4x+13}+\frac92\ln\left|(x+2)+\sqrt{x^2+4x+13}\right|+C.

Check: with t=x+2t=x+2, the derivative of t2t2+9\frac t2\sqrt{t^2+9} is 2t2+92t2+9\frac{2t^2+9}{2\sqrt{t^2+9}} and the …

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