In proportion, the product of two (inner terms) is equal to the product of two (outer terms). This is called (the basic nature of proportion).

In proportion, two external terms are reciprocal, one of which is an internal term and the other is (reciprocal of one of the internal terms).

If A ×5= B ×4, then A: B = (4): (5)

If a = 3/7 of b, then a×(7)=b×(3)

Second, choose:

Matching 4, 1.5 and 2.5, the energy composition ratio is (b: 2.4).

A 1.5 B2.4 C5

3/4 of x = 2/3 of y and x and y are not equal to 0, then x: y = (b: 8: 9).

A.3/4 to 2/3

If 2a=3b, then a is better than B = (B: 3: 2).

One third: one half.

Three. Answer the following questions according to the ratio of 18:6=9:3:

If the first term is 18-6, what does the second term become so that the equation still holds?

If the third term ×2, how should the fourth term change to make the equation still hold?

If items 3 and 4 are ÷3, can the ratio be established? Why?

(1) (18-6): x = 9: 3, so x = 4, that is, the second term becomes 4;

(2) If the third term× 2 and the fourth term should be 3 times 2, the equation still holds.

(3) It holds. According to the basic nature of the ratio, the first and last items of the ratio are multiplied or divided by the same number (except 0) at the same time, and the ratio remains unchanged.