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Miscellaneous · Q34

Q.Evaluate ∫2x+3x2+3x+9 dx\displaystyle\int \frac{2x+3}{\sqrt{x^2+3x+9}}\,dx.

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✓ Free question

The radicand's derivative is 2x+32x+3 -- exactly the numerator, with no splitting needed. Let

u=x2+3x+9u=x^2+3x+9, du=(2x+3) dxdu=(2x+3)\,dx:

∫2x+3x2+3x+9 dx=∫u−1/2 du=2u+C=2x2+3x+9+C.\int\frac{2x+3}{\sqrt{x^2+3x+9}}\,dx = \int u^{-1/2}\,du = 2\sqrt u+C = 2\sqrt{x^2+3x+9}+C.

Check: ddx ⁣[2x2+3x+9]=2⋅2x+32x2+3x+9=2x+3x2+3x+9\dfrac{d}{dx}\!\left[2\sqrt{x^2+3x+9}\right]=2\cdot\dfrac{2x+3}{2\sqrt{x^2+3x+9}} =\dfrac{2x+3}{\sqrt{x^2+3x+9}}.

✓Final answer

∫2x+3x2+3x+9 dx=2x2+3x+9+C\int\dfrac{2x+3}{\sqrt{x^2+3x+9}}\,dx=2\sqrt{x^2+3x+9}+C

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