问题如下：
A client purchases a 6-year bond at 109.83, and the coupon rate is 8%. The coupon is paid annually. The yield-to-maturity is 6% now. If the yield changes 10bps, calculate the approximate modified duration of the bond.
选项：
A. 4.78.
B. 4.06.
C. 4.02.
解释：
A is correct.
The price of the bond, if the yield decreases by 10bps:
V（-）: N=6，FV=100，PMT=8，I/Y=5.9, PV=110.36
The price of the bond, if the yield increases by 10bps:
V（+）: N=6，FV=100，PMT=8，I/Y=6.1，PV=109.31
ApproxModDur = [PV(-) - PV(+)] / 2×（△Yield）×（PV0）=（110.36-109.31）/2×0.001×109.83 = 4.78
按计算机不对，怎么回事为什么不是直接110.35-109.31再除2？？