To put it simply, the method of cross multiplication is that the multiplication on the left side of the cross is equal to the quadratic term coefficient, the multiplication on the right side is equal to the constant term, and the cross multiplication is equal to the linear term coefficient. Cross multiplication can decompose some quadratic trinomials. The key of this method is to decompose the quadratic coefficient A into two factors A 1, the product a 1, a2 of A2, and the constant term C into two factors C65438. The product of c2 c 1 times c2, a 1c2+a2c 1 happens to be a linear term b, so it can be written directly as the result: ax 2+bx+c = (a1x+c1) (a2x+C2. And realize that it is actually the inverse process of binomial multiplication. When the first coefficient is not 1, many experiments are often needed, so be sure to pay attention to the sign of each coefficient. Basic formula: x 2+(p+q) χ+PQ = (χ+p) (χ+q) The so-called cross multiplication is factorization by using the inverse operation of the multiplication formula (x+a) (x+b) = x 2+(a+b) x+ab. For example, factorizing x 2+7x+ 12 ... The constant of the above formula 12 can be decomposed into 3×4 and 3.