Given,
u=a(logeb)i^−6j^+3k^ and v=(logeb)i^+2j^+2a(logeb)k^
For angle to be acute u⋅v>0
⇒(a(logeb)i^−6j^+3k^).((logeb)i^+2j^+2a(logeb)k^)>0
⇒a(logeb)2−12+6a(logeb)>0 ∀b>1
Let y=a(logeb)2−12+6a(logeb)
Now let logeb=t⇒t>0 as b>1
Now y=at2+6at−12>0,∀t>0
Since y>0, so a>0&D<0
⇒D=36a2+48a<0
⇒a∈(3−4,0) but a>0,
So, a∈ϕ