sin3/7π*cos2/5π-cos(-3/7π)*sin(-2/5π)
=sin3/7π*cos2/5π+cos(3/7π)*sin(2/5π)
=sin(3/7π+2/5π)
=sin(49π/35)
=-sin(14π/35)
=-sin(2π/5)
sin3/7π*cos2/5π-cos(-3/7π)*sin(-2/5π)
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