ed为什么等于4呀，求解答

问题如下：

A position is worth USD 1.5 million. A two-basis-point increase in all rates causes the value to decline by USD 1199.85 and a two-basis-point decrease in all rates cause the value to increase by USD 1200.15. Estimate the effective duration and effective convexity.

选项：

解释：

Effective duration is

0.5 ×(1,199.85 +1,200.15)/(1,500,000×0.0002)=4

Also: P⁺+P^－－2P= (P⁺－P)－(P－P^－) = 1200.15－1199.85 = 0.3

so that effective convexity is 0.3/(1,500,000×0.0002²)=5

题目问：有一个头寸价值USD1.5m，利率上升2bp，使得价值下降了USD1199.85，利率下降2bp，价值上升了1200.15，请计算effective duration和effective convexity。

effective duration=0.5*(V⁺ + V^-)/(V0*2bp)

=0.5 ×(1,199.85 +1,200.15)/(1,500,000×0.0002)=4

P⁺+P^－－2P= (P⁺－P)－(P－P^－) = 1200.15－1199.85 = 0.3

effective convexity = 0.3/(1,500,000×0.0002²)=5