The substitution method is to change another algebraic expression of the independent variable into a letter, and then use this letter to represent the letter in the original independent variable.

Solution: let t= radical x+ 1, then the square of X = (T- 1), t≥ 1, and substitute it into the original formula, and you will get

Then change the letters you added back to the original independent variable.

F(t)=(t- 1) squared +2(t- 1)=t squared-1(t≥ 1),

∴f(x)=x squared-1(x≥ 1).

Matching method is to make an expression on the right side of the equation with the same algebraic expression format as the left independent variable.

Solve by collocation method: the square of f (radical x+ 1)= (radical x) +2 radical X+1.

= (radical x+ 1) de square-1,

Then according to the theorem of that identity, we can find out.

∴ f (x) = the square of x-1(x≥ 1)

Finally, why change (root number x+ 1) to x?