# The differential equation of the family of curves y = p cos (ax) + q sin (ax), where p, q are arbitrary constants, is

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The differential equation of the family of curves y = p cos (ax) + q sin (ax), where p, q are arbitrary constants, is
1. $\frac{{{d^2}y}}{{d{x^2}}} - {a^2}y = 0$
2. $\frac{{{d^2}y}}{{d{x^2}}} - ay = 0$
3. $\frac{{{d^2}y}}{{d{x^2}}} + ay = 0$
4. $\frac{{{d^2}y}}{{d{x^2}}} + {a^2}y = 0$
5. None of these

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Correct Answer - Option 4 : $\frac{{{d^2}y}}{{d{x^2}}} + {a^2}y = 0$

Calculation:

Given:

y = p cos (ax) + q sin (ax)       ….(1)

Now differentiating both sides, we get

$\Rightarrow \frac{{{\rm{dy}}}}{{{\rm{dx}}}} = {\rm{}} - {\rm{pa\;}}\sin \left( {{\rm{ax}}} \right) + {\rm{qa\;}}\cos \left( {{\rm{ax}}} \right)$

Again differentiating both sides, we get

$\Rightarrow \frac{{{{\rm{d}}^2}{\rm{y}}}}{{{\rm{d}}{{\rm{x}}^2}}} = {\rm{}} - {\rm{p}}{{\rm{a}}^2}\cos \left( {{\rm{ax}}} \right) - {\rm{q}}{{\rm{a}}^2}{\rm{\;}}\sin \left( {{\rm{ax}}} \right) = - {{\rm{a}}^2}\left( {{\rm{p\;cos\;}}\left( {{\rm{ax}}} \right){\rm{}} + {\rm{\;q\;sin\;}}\left( {{\rm{ax}}} \right)} \right)$

From equation 1st,

$\Rightarrow \frac{{{{\rm{d}}^2}{\rm{y}}}}{{{\rm{d}}{{\rm{x}}^2}}} = {\rm{}} - {{\rm{a}}^2}{\rm{y\;}}$

$\therefore \frac{{{{\rm{d}}^2}{\rm{y}}}}{{{\rm{d}}{{\rm{x}}^2}}} + {\rm{\;}}{{\rm{a}}^2}{\rm{y}}$