（3mn+1)(3mn-1)-(3mn-2)²和(2/5mn³-m²n²+1/6

解 (3mn+1)(3mn-1)-(3mn-2)²
=(3mn)²-1²-[(3mn)²-2×3mn×2+2²]
=9m²n²-1-(9m²n²-12mn+4)
=9m²n²-1-9m²n²+12mn-4
=12mn-5
(2/5mn³-m²n²+1/6n的4次方)/(2/3n²)
=[(2mn³/5)-m²n²+(n^4/6)]×3/(2n²)
=(2mn³/5)×3/(2n²)-m²n²×3/(2n²)+(n^4/6)×3/(2n²)
=(3mn/5)-(3m²/2)+(n²/4)

顶楼上！