Explain by mirror image method: The parabolic opening is upward, and the following conditions need to be met to make it always greater than zero on the positive semi-axis of X:

Symmetry axis is less than or equal to 0; The larger of the two roots of the equation is less than or equal to zero. that is

-2a/2 & lt； =0

Formula for finding the root (root sign is not allowed)

Select d as the calculation result.