(a+2b)·(a-3b)
=a^2-a·b-6b^2
=0
a·b=-2
(a+kb)·(5a+b)
=5a^2+(1+5k)a·b+kb^2
=5a^2+(1+5k)|a|*|b|*cos120+kb^2
=80+(1+5k)4*2*(-1/2)+4k
=0
k=19/4
(a^2=(|a|)^2,代入计算即可)
(a+2b)·(a-3b)
=a^2-a·b-6b^2
=0
a·b=-2
(a+kb)·(5a+b)
=5a^2+(1+5k)a·b+kb^2
=5a^2+(1+5k)|a|*|b|*cos120+kb^2
=80+(1+5k)4*2*(-1/2)+4k
=0
k=19/4
(a^2=(|a|)^2,代入计算即可)