a) tự làm nha
b xét tích ac ta có: \(-m^2+m-1=-\left(m^2-m+\frac{1}{4}+\frac{3}{4}\right)=-\left[\left(m-\frac{1}{2}\right)^2+\frac{3}{4}\right]\)
ta có: \(\left(m-\frac{1}{2}\right)^2\ge0\Leftrightarrow\left(m-\frac{1}{2}\right)^2+\frac{3}{4}>0\Rightarrow-\left[\left(m-\frac{1}{2}\right)^2+\frac{3}{4}\right]