(1)mx-2x+3=0有实根 ∴判别式△=(-2)-4m×3=4-12m≥0 ∴m≤1/3 (2)x+2(m-2)x+m+4=0的两根平方和为21 ∵x1+x2=2(2-m),x1x2=m+4 ∴x1+x2 =(x1+x2)-2x1x2 =(4-2m)-2(m+4) =16-16m+4m-2m-8 =2m-16m+8 ∴2m-16m+8-(m+4)=21 即m-16m-17=0 ∴m=-1或m=17