関連ワード:  if tangent is parallel to x axis    if tangent is parallel to x axis then dy dx    if tangent is parallel to x axis then    when tangent is parallel to x axis    if tangent is perpendicular to x axis    if tangent is perpendicular to x axis then dy dx    when tangent is perpendicular to x axis    if tangent is parallel to x axis what is the slope    slope when tangent is parallel to x axis    gradient when tangent is parallel to x axis