Test Question $\mathbf{A}$ Simplify
$$
\frac{a^{2}(x-b)(x-c)}{(a-b)(a-c)}+\frac{b^{2}(x-c)(x-a)}{(b-c)(b-a)}+\frac{c^{2}(x-a)(x-b)}{(c-a)(c-b)} \text {. }
$$

Its simplest result is $\qquad$ (1990 National Junior High School "Lefeng Cup" Mathematics Invitational Competition)
(Solution: $x^{2}$ )