cosa=-3/5,a属于（π/2,π)
sina=4/5
sin2a
=2sinacosa
=2*(4/5)*(-3/5)
=-24/25
cos2a
=2cos^2a-1
=2*(-3/5)^2-1
=18/25-1
=-7/25
tan2a
=sin2a/cos2a
=(24/25)/(-7/25)
=-24/7