原式=(a-3b)²/a(a-2b)÷(5b²-a²+4b²)/(a-2b)-1/a=(a-3b)²/a(a-2b)×(a-2b)/(3b+a)(3b-a)-1/a=-(a-3b)/a(a+3b)-1/a=-(a-3b+a+3b)/a(a+3b)=-2a/a(a+3b)=-2/(a+3b)