a=e1-2e2
,b=ke1+e2
a*b
=(e1-2e2)(ke1+e2)
=k(e1)²+(1-2k)e1e2-2(e2)²
=k+(1-2k)*(-1/2)-2
=k-(1/2)(1-2k)-2
=k+k-5/2=0
得 2k=5/2
所以
k=5/4