2. There are three pictures left besides the radicals. The stroke order of concave characters is vertical, horizontal folding, vertical, horizontal folding and horizontal.

3. concave: āo? . Describe the surrounding high and the middle low.

(1) Gravure: Ob γ n: A printing plate in which characters or lines carved on stones or other hard materials are recessed?

(2) Concavity: Curvature ā od? concave surface?

(3) concave arc decoration: decorative lines about a quarter of the circumference of the concave arc?

(4) concave mirror: concave spherical mirror, when parallel light shines on it, it is focused on the front of the mirror through its reflection?

(5) concave lens: concave lens (the center is thinner than the edge) has the effect of diverging light?

(6) Bump āotū: Not smooth?

(7) Depression: high around and low in the middle.