The difference between them lies in the understanding of stem and physical problems, because the value of gravitational constant G in the universal gravitation formula corresponds to the international system of units, that is, the units of M and M in the formula are both kg.

Only "unit mass" is mentioned in the stem, but it is actually relative mass. Now the difference between these two pictures is obvious:

In the first picture, the unit mass is directly regarded as 1kg, and the disc mass is directly regarded as μdσ kg, which is directly substituted into the formula.

The second picture closely follows the topic and thinks that the unit mass is a relative value. For example, taking the mass of the earth as a reference, the unit mass is the mass of the earth, so he needs to assume that the unit particle mass is M kg, and the actual mass of the disk is m kg, so that the actual mass of the disk is infinitely small1* dσ/(π A2/2) * m kg, and the result of the second picture can be obtained by substituting it into the formula of universal gravitation. However, there is a clerical error in the first formula in the figure, and the denominator π/2 should be π/2a 2.

Comprehensive evaluation, the second picture is more rigorous, of course, you can also think that the second picture is too dead. These two pictures give me the feeling that the first one is an example of calculus in the current college classroom, and it does not seek an extraordinary solution; The second is an example of calculus in an older textbook, which is full of pedantry.