证明：∵2x+x-3 =2(x+1/2x)-3 =2×[x+2×1/4×x+(1/4)-(1/4)]-3 =2[(x+1/4)-1/16]-3 =2(x+1/4)-1/8-3 =2(x+1/4)-25/8 ∵不论x为何实数,均有(x+1/4)≥0 ∴2(x+1/4)-25/8≥-25/8 即原代数式的值不小于-25/8...