✔ 最佳答案
Since force F equals to the negative of the potnetial gradient,
i.e. F = -dU/dx
hence, on the right hand side of the origin (region of +ve x), the slope of the curve is +ve. Hence, the force F is -ve. That means the force is directed towards the -ve x direction, i.e towards the origin.
On the left hand side of the origin (i.e. region of -ve x), the slope is -ve, hence -dU/dx is +ve. That means the force F is in the direction of +ve x, i.e. towards the origin. The force is thus a restoring force.
Syummetry alone is not a sufficient condition to qualify a restoring force. It only shows that the forces are equal on both sides of the origin.