Question

初中一年级数学计算题一道

Answer 1

两个非负数之和为零，则它们都等于零，易求得m=5/2，n=5，把所求代数式化简后再代入便可求值。

Answer 2

解:2m-5=0
m=5/2
2m-5n+20=0
n=5
(-2m^2)-2m(5n-2m)+3n(6m-5n)-3n(4m-5n)
=-2m^2-10mn+4m^2+18mn+15n^2-12mn+15n^2
=2m^2-4mn+30n^2
=2(m^2-2nm+15n^2)
=2x(9/4-25+375)
=4.5-50-750
=-795.5

Answer 3

因为|2m-5|+(2m-5n+20)^2=0
所以，
2m-5=0
2m-5n+20=0
m=5/2
n=5
原式＝-2m^2-10mn+4m^2+18mn-15n^2-12mn+15n^2
=2m^2-4mn
=2×(5/2)^2-4×5/2×5
=-75/2

Answer 4

Answer 5

2m-5和2m-5n+20为零，剩下不难