Answer :
To solve the equation
$$
4x^4 - 21x^2 + 20 = 0,
$$
we can make a substitution to reduce it to a quadratic form.
1. Notice that the term $x^4$ can be expressed as $(x^2)^2$.
2. Let
$$
u = x^2.
$$
3. Then, the equation becomes:
$$
4u^2 - 21u + 20 = 0,
$$
which is a quadratic equation in $u$.
Thus, the substitution that should be used is
$$
u = x^2.
$$
$$
4x^4 - 21x^2 + 20 = 0,
$$
we can make a substitution to reduce it to a quadratic form.
1. Notice that the term $x^4$ can be expressed as $(x^2)^2$.
2. Let
$$
u = x^2.
$$
3. Then, the equation becomes:
$$
4u^2 - 21u + 20 = 0,
$$
which is a quadratic equation in $u$.
Thus, the substitution that should be used is
$$
u = x^2.
$$
