In our primary school mathematics knowledge system, some mathematics problems are often difficult to solve, and even fall into a "dead game". If we only consider the known conditions, we can find out the desired results. If we can change the angle of thinking, proceed from the result of stating things, and use known conditions to carry out backward reasoning step by step, and gradually approach what we want until we solve the problem, this thinking method is usually called backward deduction, also called backward deduction or reduction.

We always have a mindset when solving problems, that is, "the result of condition orientation". Often used to proceed from the existing conditions, how many conditions, how many things to do, that is to say, conditions determine the outcome. If we speculate from the back to the front with the expected goal, you will find that many problems will be solved. Reverse deduction also plays an important role in solving primary school mathematics problems. There are two common problem-solving strategies: algorithm inversion and graphic inversion.

First, algorithm inversion.

The solution of the problem is usually: that is to say, the original is addition, but it is subtraction; It turned out to be subtraction, and vice versa; Similarly, the original is multiplication, and the reverse is division; It turned out to be division, and vice versa.

Second, the graphics are inverted.

Graphic inversion method can help solve complex application problems and difficult reasoning problems with the help of "line chart", "flow chart" and "list method"