原式=2/3×a-2/3×1/2b-2/3×c-3/4×2/3a-3/4×b-3/4×(-1/3c)
=2/3a-1/3b-2/3c-1/2a-3/4b+1/4c
=(2/3a-1/2a)+(-1/3b-3/4b)+(-2/3c+1/4c)
=1/6a-13/12b-5/12c