解由题可得1.5a+0.75(a-b)+0.5(a+c)
=1.5a+0.75a-0.75b+0.5a+0.5c
=2.75a-0.75b+0.5c(km)
答汽车一共行驶(2.75a-0.75b+0.5c)千米