√(x²+p²)-p
=[√(x²+p²)-p][√(x²+p²)+p]/[√(x²+p²)+p]
=[(x²+p²)-p²]/[√(x²+p²)+p]
=x²/[√(x²+p²)+p]
分子趋于0
分母趋于(√p²+p)
所以极限=0