|a-t*e|>=|a-e|
|a-t*e|^2>=|a-e|^2
a^2-2ate+t^2e^2≥a^2-2ae+e^2
-2ate+t^2≥-2ae+1
t^2-2aet+2ae-1≥0 对任意t属于R成立
△=4a^2e^2-4(2ae-1)=4a^2e^2-8ae+4=4(ae-1)^2≤0
ae=1
因为e^2=1,所以
ae=e^2
ae-e^2=0
e(a-e)=0
e⊥(a-e)
（C）e⊥(a-e)