(A C): (B C) = (A C) * (C) = A B.

That is, (a/c):(b/c)=a:b

(2) Bank acceptance bills and letters of credit

b/a = 1/(a/b)= 1/(c/d)= d/c

That is, b/a = d/c.

(that is, they are all backwards and still equal)

(3)(a+b)/b and (c+d)/d

(a+b)/b = a/b+b/b = a/b+ 1 = c/d+ 1 = c/d+d/d =(c+d)/d

That is, (a+b)/b = (c+d)/d.

(Similarly (a+b)/a=(c+d)/c (Prepare for the next question))

(4)(a+b)/(a-b) and (c+d)/(c-d)

(a≠b，c≠d)

Because (a+b)/b=(c+d)/d and (a+b)/a = (c+d)/c.

According to the conclusion of (2),

So there are b/(a+b)=d/(c+d) and a/(a+b)=c/(c+d).

Subtraction of two equations

So a/(a+b)-b/(a+b)=c/(c+d)-d/(c+d)

That is, (a-b)/(a+b)=(c-d)/(c+d)

According to the conclusion of (2),

Have (a+b)/(a-b)=(c+d)/(c-d)