Question 1086576
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<pre>
If  {{{tan(2*theta) * tan(3*theta)}}} = 1   then

    {{{2*theta}}} = {{{pi/2}}} - {{{3*theta)}}},

    which implies  {{{5*theta)}}} = {{{pi/2}}},

    and then  {{{cos(5*theta)}}} = 0.
</pre>