（1）6a²b³/-8ab⁴
=-3a/(4b)
(2）5a²(b-c)³/10a³(c-b)²
=(b-c)/(2a)
(3)(m-1)(m-2)/m²-m
=(m-1)(m-2)/[m(m-1)]
=(m-2)/m
(4)8(y-x)⁴/y(x-y)
=-8(y-x)³/y
(5) x²-4/(x+7)(x-2)
=(x+2)(x-2)/[(X+7)(X-2)]
=(x+2)/(x+7).