A curve is concave up where it cups upward (like a smile) and concave down where it cups downward (like a frown).
Test
- f''(x) > 0 — concave up.
- f''(x) < 0 — concave down.
- f''(x) = 0 — possible inflection point.
A curve is concave up where it cups upward (like a smile) and concave down where it cups downward (like a frown).