for any equation p(x)=0 (where p(x) is an expression or function of x) has roots a0, a1,....then at x=a0, a1,... p(x) becomes 0. in your example, p(x)=1 - x^2/4. At x=2 or -2, i.e., p(2)=p(-2) = 0.

## for any equation p(x)=0

for any equation p(x)=0 (where p(x) is an expression or function of x) has roots a0, a1,....then at x=a0, a1,... p(x) becomes 0. in your example, p(x)=1 - x^2/4. At x=2 or -2, i.e., p(2)=p(-2) = 0.