(3向量a+5向量b)⊥(m向量a-向量b)
=>(3向量a+5向量b)(m向量a-向量b)=0
=>3ma²-3ab+5mab-5b²=27m+(5m-3)lallblcos60°-20=27m+15m-9-20=0
=>m=29/42