WAEC 2020 · Paper 1 · Q27

Which of the following is not an equation of a circle?

Worked solution (try it first)

In a circle's equation the $x^2$ and $y^2$ terms have the same coefficient (and the same sign), and there is no $xy$ term.
A, B and C all have equal coefficients of $x^2$ and $y^2$ (3 and 3, 1 and 1, 5 and 5), and each has a positive radius squared.
In D the coefficients are $1$ and $-1$ , so it is not a circle (it is a hyperbola).
The answer is option D.