f(t)=(-cos^2t+4sint+6)/(sint-3)
=[-(1-sint^2)+4sint+6]/(sint-3)
=(sint^2+4sint+5)/(sint-3)
所以f(t)就是点A(sint,sint^2+4sint)和点B(3,-5)所在直线的斜率
点A就是二次函数y=x^2+4x,当-1