a=2t b=3t c=4t
(a+3b-2c)/(2a-b)
=(2+9-8)t/(4-3)t
=3
(a+3a-2c)/(2a-b)
=(2+6-8)t/(4-3)t
=0
之所以列两个式子,是怀疑你+3a应为+3b