切り詰めた円錐の表面積を計算するように関数を修正するには、関数に別の計算ステップを追加します。以下は、表面積を計算するために前の関数をどのように修正するかの例です：
``` 
def truncated_cone(radius1, radius2, height):
    """
    Calculates the volume and surface area of a truncated cone.

    Args:
        radius1 (float): The radius of the larger base.
        radius2 (float): The radius of the smaller base.
        height (float): The height of the truncated cone.

    Returns:
        tuple: A tuple containing the volume and surface area of the truncated cone.

    Raises:
        ValueError: If the input values are not positive numbers.

    """
    # Check input values are positive numbers
    if radius1 <= 0 or radius2 <= 0 or height <= 0:
        raise ValueError("All input values must be positive numbers")

    # Calculate volume of truncated cone using formula
    volume = (1/3) * height * (radius1**2 + radius2**2 + radius1 * radius2)

    # Calculate surface area of truncated cone using formula
    slant_height = math.sqrt((radius1 - radius2)**2 + height**2)
    lateral_area = math.pi * slant_height * (radius1 + radius2)
    base_area1 = math.pi * radius1**2
    base_area2 = math.pi * radius2**2
    surface_area = lateral_area + base_area1 + base_area2

    return (volume, surface_area)
```
この修正された関数では、切頭円錐の表面積を計算する新しい計算ステップを追加しました。計算式を使って、傾斜の高さ、横方向の面積、大きい底辺と小さい底辺の面積を計算しました。そして、これらの面積を合計して、切り詰めた円錐の総表面積を求めました。

この関数が切り詰めた円錐の体積と表面積の両方を含むタプルを返すようになったことを反映するために、docstringも更新したことに注意してください。また、sqrt関数を使用して斜めの高さを計算するために、mathモジュールをインポートしました。
To modify the function to calculate the surface area of the truncated cone, you can add another calculation step to the function. Here's an example of how you can modify the previous function to calculate the surface area:
``` 
def truncated_cone(radius1, radius2, height):
    """
    Calculates the volume and surface area of a truncated cone.

    Args:
        radius1 (float): The radius of the larger base.
        radius2 (float): The radius of the smaller base.
        height (float): The height of the truncated cone.

    Returns:
        tuple: A tuple containing the volume and surface area of the truncated cone.

    Raises:
        ValueError: If the input values are not positive numbers.

    """
    # Check input values are positive numbers
    if radius1 <= 0 or radius2 <= 0 or height <= 0:
        raise ValueError("All input values must be positive numbers")

    # Calculate volume of truncated cone using formula
    volume = (1/3) * height * (radius1**2 + radius2**2 + radius1 * radius2)

    # Calculate surface area of truncated cone using formula
    slant_height = math.sqrt((radius1 - radius2)**2 + height**2)
    lateral_area = math.pi * slant_height * (radius1 + radius2)
    base_area1 = math.pi * radius1**2
    base_area2 = math.pi * radius2**2
    surface_area = lateral_area + base_area1 + base_area2

    return (volume, surface_area)
``` 
In this modified function, we added a new calculation step to compute the surface area of the truncated cone. We used the formula to calculate the slant height, the lateral area, and the base areas of the larger and smaller bases. Then, we added up these areas to get the total surface area of the truncated cone.

Note that we also updated the docstring to reflect that the function now returns a tuple containing both the volume and surface area of the truncated cone. We also imported the math module to use the sqrt function to calculate the slant height.