A square has perimeter 36. What is its area?

Square: all four sides equal. Perimeter = 4 × side, so 36 = 4s, giving s = 9. Area = s² = 9² = 81 square units. Square properties to know: (1) All sides equal; (2) All angles 90°; (3) Diagonals are equal in length and bisect each other at 90°; (4) Diagonal length = s√2 (Pythagorean theorem with two sides of length s); (5) Perimeter = 4s; (6) Area = s²; (7) A square is a special rectangle, rhombus, and parallelogram. Don't confuse perimeter and area: perimeter is linear (boundary length), area is square (surface). Common error: dividing perimeter by 2 instead of 4 (forgetting all four sides), or finding side then forgetting to square it. Always identify what the problem asks for. Two-step problems are common on ASVAB MK: extract one quantity, use it to find another.