Question

八年级数学题（详细过程）

已知m-2n=3.求[(3m+2n)(3m-2n)-(m+2n)(5m-2n)]÷1/3 m 的值。

已知 2a-b=5.求[(a²+b²)+2b(a-b)-(a-b)²]÷4b的值

Answer 1

1）先化简 9m²-4n²-5m²+2mn-10mn+4n²=(4m²-8mn)÷1/3 m=4m(m-2n)× 3/m
又因为已知m-2n=3. 所以=3× 4× 3=36
2）化简 [a²+b²+2b(a-b)-(a-b)²]÷4b 配方 [(b+a-b)²+a²-2(a-b)²]÷4b=[a²+a²-2(a²+b²-2ab)]÷4b
=b(2a-b)÷4b 已知 2a-b=5 所以=5/4

Answer 2

[(3m+2n)(3m-2n)-(m+2n)(5m-2n)]÷(1/3)m
=[9m²-4n²-5m²+2mn-10mn+4n²]÷(1/3)m
=[4m²-8mn]÷(1/3)m
=4m(m-2n)÷(1/3)m
=12*(3)
=36
[(a²+b²)+2b(a-b)-(a-b)²]÷4b
=[a²+b²+2ab-2b²-a²+2ab-b²]÷4b
=[4ab-2b²]÷4b
=2b(2a-b)÷4b
=2*(5)÷4
=5/2