"Cross multiplication" in mathematics is generally used for factorization of quadratic trinomial, such as 2x2+7x+3;

We decompose quadratic coefficient 2 into two factors 1 and write it on the left, and constant coefficient 3 into two factors 3, 1 and write it on the right (as shown below).

^^^^^ 1,^^^3,

^^^^^2,^^^ 1,

Their diagonal line is cross. Multiply them and add them:

1* 1+2*3= 1+6=7; Exactly equal to the coefficient of the first term,

Then 2x2+7x+3; =( 1X+3)(2X+ 1)=(X+3)(2X+ 1)

This is also an effective and fast method to solve the quadratic equation of one variable. The key is your proficiency.