证明：-10x²+7x-4
　　=-10(x²+7x/10-49/400)-4+49/40
　　=-10(x-7/20)²-111/40
　　∵(x-7/20)²≥0
　　∴-10(x-7/20)²≤0
　　∴-(x-7/20)²-111/40