I then demonstrate algebraically that if the variable of interest, be it financial depth, doctors, or any other good or service that rises along with per capita income, is incorporated in a quadratic form into a regression of growth on per capita income, there will be a necessary but spurious finding that above a certain point more of the good or service in question causes growth to decline.

Solve by three methods—formula, factoring, and completing the square: Review equations in the quadratic form and solve: 2.

Equations in the quadratic form.