What is the product of the reciprocal of 8 / 9 and the reciprocal of 3 / 4?

What is the product of the reciprocal of 8 / 9 and the reciprocal of 3 / 4?

1 / (8 / 9) × 1 / (3 / 4)
=9 / 8 × 4 / 3
=3 / 2

Calculate the product of (1 + 2 / 1) * (1 + 4 / 1) * (1 + 6 / 1) * (1 + 8 / 1) * (1 + 3 / 1) * (1 + 5 / 1) * (1 + 7 / 1) * (1 + 9 / 1)

In fact, (1 + 1 / 2) * (1 + 1 / 3) * (1 + 1 / 4) * * (1 + 1 / 9) = (3 / 2) * (4 / 3) * (5 / 4) * * (10 / 9) = 5
The numerator of the former can be reduced to the denominator of the latter

Why is the density of kerosene in the book 0.8x10 ^ 3 instead of 8x10 ^ 2? 0.8x10 ^ 3 is not a scientific counting method. I think 8x10 ^ 2 is the standard

Your idea is right, but do you know that the unit of density is kg / m3 in the main unit of international unit and the commonly used unit G / cm3, 31000kg / m3 = 1g / cm3; so one advantage of doing this is that when the density value in the density meter is converted to g / cm3, you only need to take the third power of 10
Of course, this convenience does not apply to the density of the gas

Two point charges in vacuum, Q1 = 8x10 ^ - 9C and Q2 = - 1.8x10 ^ - 8C, are fixed at 20c apart
Two point charges in vacuum, Q1 = 8x10 ^ - 9C and Q2 = - 1.8x10 ^ - 8C, are fixed on two points a and B 20cm apart, as shown in the figure. If a point charge is placed at a certain point on the line a and B (or extension line) and can just be stationary, what is the position of this point?

F = kq1q2 / R ^ 2, we can see that the third point charge should be placed on the side of Q1, and let the distance Q1 be r
Q1/r^2=Q2/(r+0.2)^2
Substituting Q1 and Q2, r = 0.4m