tg(3π-α)=3/4,
-tgα=3/4
tgα=-3/4
α∈(π/2,3π/2),
则：α∈(π/2,π)
sinα=(-3/4)cosα
sin²α+cos²α=1
(25/16)cos²α=1
cosα=-4/5
sinα=3/5
sin2α=2sinαcosα=-24/25
cos2α=2cos²α-1=7/25
所以,
cos(π/2+α)=-sinα=-3/5
sin(π/4+2α)=√2/2(cos2α+sin2α)
=√2/2(7/25-24/25)
=-17√2/50