如图所示,设M是△ABC的重心,过M的直线分别交边AB,AC于P,Q两点,且AP/PB=m,AQ/QC=n,则1/m+1/n=_.

问题描述:

如图所示,设M是△ABC的重心,过M的直线分别交边AB,AC于P,Q两点,且

AP
PB
=m,
AQ
QC
=n,则
1
m
+
1
n
=______.

分别过点B,C作BE∥AD,CF∥AD,交PQ于点E,F,则BE∥AD∥CF,
∵点D是BC的中点,
∴MD是梯形的中位线,
∴BE+CF=2MD,

1
m
+
1
n
=
PB
AP
+
CQ
AQ
=
BE
AM
+
CF
AM
=
BE+CF
AM
=
2MD
AM
=1.