f(x)=(1-sinx+cosx) / cosx
分母不为零：
cosx ≠ 0
定义域：x ≠ kπ ± π/2 ( k∈Z)
a是第四象限角
tana=-4/3
cosa = 1/根号[1+tan^2a] = 1/根号[1 + (-4/3)^2 ] = 3/5
f(a)=(1-sina+cosa) / cosa
=1/cosa - tana + 1
=1/(3/5) -(-4/3) +1
=4