a·b
=3cosx-4sinx
=5cos(x+t)
a·b∈[-5,5]
|向量a-2向量b|²
=|a|²-4a·b+4|b|²
=25-4a·b+4
=29-4a·b∈[9,49]
|向量a-2向量b|∈[3,7]