✔ 最佳答案
設L為通過(0,0,1)與(1,2,5)兩點之直線,則
(1) X軸上距離L最近之點為何?
(2) X軸與L之距離為何?
Sol
(1)
設A(a,0,0) 為X軸上距離L最近之點,(e,f,g)為L上最接近 X軸之點
L:x/1=y/2=(z-1)/4
e/1=f/2=(g-1)/4=p
e=p,f=2p,g=4p+1
|i j k|
|1 2 4|=4j-2k
|1 0 0|
e-a:f:g=0:2:-1
a=e,f=-2g
d^2=(e-a)^2+f^2+g^2
=(2p)^2+(4p+1)^2
=20p^2+8p+1
f(p)=20p^2+8p
f’(p)=40p+8
Set f’(p)=0
p=-1/5
a=e=-1/5
A(-1/5,0,0)
(2)
d^2=20p^2+8p+1=20*(1/25)+8*(-1/5)+1=0.2=5/25
d=√5/5