已知a=3b,c=4a
则b=a/3 c=4a代入
(2a-9b+2c)分之(5a+6b-c)
=[5a+6*(a/3)-4a]/[2a-9*(a/3)+2*4a]
=(a+2a)/(2a-3a+8a)
=3a/(7a)
=3/7