629. Investigate the convexity and concavity of the curve $y=$ $=x^{2}-x$.
Solution. $1^{0}$. Find the second derivative: $y^{\prime}=2 x-1 ; y^{\prime \prime}=2$.

$2^{0}$. Since the second derivative is positive for any $x$, the curve is concave up on the entire domain ( $-\infty, \infty$ ).