The formula is always valid, and the profit and loss account is called "transitional account", so it is carried forward to other accounts at the end of the period, but the formula is still valid because the beginning and the end of the period can be regarded as "zero". If you substitute 0 into the formula, you will find that the formula is still correct. In principle, except for profit and loss subjects, there is a balance at the end of the period, but it is not uncommon for the balance to be "zero" because there is no related business.

It can be said that the formula of "opening balance+current amount-current decrease = final balance" is always valid for any account (including accounts without opening balance and final balance). Accounts without opening and closing balances are mainly profit and loss accounts.

There is no balance after the end of the period, such as main business income, other business income, investment income, non-operating income, main business cost, other business cost, business tax and surcharge, sales expenses, management expenses, accounting expenses, non-operating expenses, income tax expenses, etc.

Accounting equation is a mathematical expression that reflects the economic relationship of various accounting elements in accounting, also known as accounting equation, accounting balance formula and accounting identity. It is the theoretical basis for each accounting entity to set up accounts for double-entry bookkeeping and prepare accounting statements.