Method of substitution is a very important and widely used method to solve problems in mathematics. We usually refer to unknowns or variables as elements. The so-called method of substitution is to replace a part of the original formula with new variables in a complicated mathematical formula, thus simplifying it and making the problem easy to solve. Method of substitution can be used to solve higher order equations when solving quadratic equations with one variable.

For example, solving equation (x? - 1)? -5(x？ - 1)+4=0,

We can put (x? -1) as a whole, and then set x2- 1=y, then the original equation can be transformed into y? -5y+4=0，

The solution is y 1= 1, and y2 = 4.

When y= 1 - 1= 1,∴x2=2,∴x= 2；

When y=4, x? -1= 4, ∴ x2 = 5, ∴ x = No.5,

So the solution of the original equation is x 1= 2, x2=-2, x3=5, x4 =-5.