What are some examples of mathematical concepts which are very intuitive but difficult to prove mathematically? No answer needed!

Measures of the angle of r, 31 s (x+4) t (3x+9)

Use composition to determine if (x) or h(x) is the inverse of f(x) for the limited domain x2-1.

Scenario: a rectangular plot of ground is 5 meters longer than it is wide. its area is 20,000 square meters. question: what equation will you find the dimensions? note: let w represent the width. options: w(w+5)=20,000 w^2=20,000+5 (w(w+5))/2=20,000 w+2(w+5)=20,000