(1/2a+1/3b)/(2/3a-1/4b)
=（a/2+b/3)/(2a/3-b/4)
=(3a/6+2b/6)/(8a/12-3b/12)
=[(3a+2b)/6]/[(8a-3b)/12]
=[(3a+2b)/6]*[12/(8a-3b)]
=(6a+4b)/(8a-3b)