[Euler-Fermat]

3.$\sin(3x)+\cos(3x) = 0.25$
$1-2\sin(3x)\cos(3x) = \frac{1}{16}$
$\sin(6x) = \frac{15}{16} $
$0\leqslant x\leqslant 9\pi $
$0\leqslant 6x \leqslant 54\pi $
$\therefore $มี $54$ คำตอบ