原式=
(2a的n+5次方-3a的n+4次方-4a的n+3次方)除以(-5a的n-1次方)
=2a的n+5次方÷-5a的n-1次方-3a的n+4次方÷-5a的n-1次方-4a的n+3次方÷-5a的n-1次方
=-(2 a^6)/5+(3 a^5)/5+(4 a^4)/5