设a=(1,0)则b=(-1/2,√3/2)
c=3a+5b=(1/2,5√3/2),d=ma-3b=(m+3/2,-3√3/2)
a+3b=(-1/2,3√3/2) |a+3b|=7
若c*d=0,则c⊥d
（1/2)(m+3/2）+(5√3/2)(-3√3/2) =0
m=17
c//d
(1/2)(-3√3/2)=(5√3/2)(m+3/2) m=-1.8