(x- 1)*x=990

Interpretation of meaning:

Because everyone shakes hands with others (x- 1), * * there are x people shaking hands, and every two people shake hands only once. x(x- 1) has repeated counting, so it is divided by 2.

Calculate x = 100.

Let the long side of a rectangle be x meters.

Then x (18/2-x) = 3 * 8/2 (triangle area)

X = 3 is the short side (truncation) and X = 6.