原式=(a-3b)²/a(a-2b)÷[5b²-(a²-4b²)]/(a-2b)-1/a
=(a-3b)²/a(a-2b)×(a-2b)/(a+3b)(a-3b)-1/a
=(a-3b)/a(a+3b)-1/a
=(a-3b-a-3b)/a(a+3b)
=-6b/(a²+3ab)
=-6*1/(9+9)
=-1/3