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Q.Evaluate ∫ e^(3x) cos 5x dx. OR Evaluate ∫₀² (x² + 3) dx as limit of a sum.

Punjab PsebPSEB Punjab Class 12 Board 2019Subjective· 4mImportance★★★★★
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Use integration by parts twice (or the standard reduction formula) for ∫eaxcos⁡(bx) dx\int e^{ax}\cos(bx)\,dx.

Let I=∫e3xcos⁡5x dxI = \displaystyle\int e^{3x}\cos5x\,dx.

Integrate by parts twice, taking e3xe^{3x} as the part to differentiate each time:

I=e3x⋅sin⁡5x5−35∫e3xsin⁡5x dxI = e^{3x}\cdot\frac{\sin5x}{5} - \frac35\int e^{3x}\sin5x\,dx

For ∫e3xsin⁡5x dx\displaystyle\int e^{3x}\sin5x\,dx, integrate by parts again:

∫e3xsin⁡5x dx=−e3xcos⁡5x5+35∫e3xcos⁡5x dx=−e3xcos⁡5x5+35I\int e^{3x}\sin5x\,dx = -e^{3x}\frac{\cos5x}{5} + \frac35\int e^{3x}\cos5x\,dx = -\frac{e^{3x}\cos5x}{5}+\frac35 I

Substituting back:

I=e3xsin⁡5x5−35(−e3xcos⁡5x5+35I)=e3xsin⁡5x5+3e3xcos⁡5x25−925II = \frac{e^{3x}\sin5x}{5} - \frac35\left(-\frac{e^{3x}\cos5x}{5}+\frac35 I\right) = \frac{e^{3x}\sin5x}{5}+\frac{3e^{3x}\cos5x}{25}-\frac{9}{25}I

I+925I=e3xsin⁡5x5+3e3xcos⁡5x25I + \frac{9}{25}I = \frac{e^{3x}\sin5x}{5}+\frac{3e^{3x}\cos5x}{25} …

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