因为[（a+2b）/（a-2b）]=2
所以[（a+2b）/（a-2b）]+[（3a-6b）/（4a+8b）]
=2+[3(a-2b)]/[4(a+2b)]
=2+3/4×（a-2b）/(a+2b)
=2+3/4×1/2
=2+3/8
=19/8