For personal finance (even though this question is tagged for stocks):
How about the fundamental accounting equation?
Assets = Liabilities + Equity
Equity = Assets - Liabilities
In a personal finance context, "equity" is better known as "net worth"
The concept of net present value
The actual equation is a little ...
For personal finance, the Micawber Principle, from Charles Dickens' David Copperfield:
Annual income twenty pounds, annual expenditure nineteen, nineteen and
six, result happiness. Annual income twenty pounds, annual
expenditure twenty pounds ought and six, result misery.