已知,|2m-5|+(2m-5n+20)^2 = 0 ,且绝对值和平方值都大于等于0,可得：2m-5 = 0 ,2m-5n+20 = 0 ,解得：m = 5/2 ,n = 5 .(-2m)^2-2m(5n-2m)+3n(6m-5n)-3n(4m-5n)= 4m^2-10mn+4m^2+18mn-15n^2-12mn+15n^2= 8m^2-4mn= 4m(...