4.曲线 =cgx 在点 (dfrac (pi )(4),1) 处的法线方程为 ()-|||-A. -1=-2(x-dfrac (pi )(4)); B. -1=dfrac (1)(2)(x-dfrac (pi )(4)); C. -1=-dfrac (1)(2)(x-dfrac (pi )(4)); D. -1=2(x-dfrac (pi )(4))