y=x^3导数为y=3x^2,直线与其切点为（m,m^3）
则直线过（m,m^3）,(1,0)
求得直线为y=0或者y=27/4*(x-1)
若y=0.则y=ax^2+15/4x-9顶点在x轴
得a=-25/64
若y=27/4*(x-1),斜率为27/4
y=ax^2+15/4x-9导数为y=2ax+15/4,
直线与其切点为（n,an^2+15/4n-9）
2an+15/4=27/4
n=3/(2a)
直线过（3/2,27/8）,(1,0) (3/(2a),(63-72a)/8a)
推出a=-1
所以a=-25/64或者a=-1
sin^2a+cos^2a=1,得sin^2a=4/5
f(x)=(1+1/tanx)sin^2-2sin(x+π/4)sin(x-π/4).
=sinx(cosx+sinx)+2sin(x+π/4)cos(x+π/4)
=1/2sin2x+sin^2x+sin(2x+π/2)
=1/2sin2x+1/2-1/2cos2x+cos2x
=(sin2x+cos2x+1)/2
tana=sina/cosa=2,所以sin^2a=4cos^2a,即1-cos2a=4*（1+cos2a）得cos2a=-3/5
sin2a=2sinacosa/（sin^2a+cos^2a）=2tana/（tan^2a+1）=4/5
所以f(a) =1/2*（4/5-3/5+1）=3/5