中文：

本金是金融学中的一个重要概念，它指的是投资者投入到一项投资中的资金。本金是一个重要的财务指标，它的变化直接影响到投资者的收益和风险。因此，了解本金的计算公式对于投资者来说非常重要。

本金的计算公式可以根据不同的投资方式和金融工具而有所不同。但是，一般来说，本金的计算公式可以表示为：本金 = 投资金额 + 相关费用。

在这个公式中，投资金额是指投资者投入到投资项目中的资金数额，而相关费用则包括交易费用、税费、管理费用等与投资相关的费用。这些费用可能会因为不同的投资项目和金融工具而有所不同。

例如，在购买一只股票时，投资者需要支付交易费用和税费等，这些费用会算入到购买成本中。因此，在计算本金时，需要将购买成本作为投资金额，并将交易费用和税费等作为相关费用进行计算。

英文：

Principal is an important concept in finance, which refers to the funds invested by an investor in a particular investment. The principal is a crucial financial indicator, and its changes directly affect the investor's收益和risk. Therefore, understanding the calculation formula of principal is very important for investors.

The calculation formula of principal can be different depending on different investment methods and financial instruments. Generally speaking, the calculation formula of principal can be expressed as: principal = investment amount + related fees.

In this formula, the investment amount refers to the amount of funds invested by the investor in the investment project, while the related fees include transaction fees, taxes, management fees, etc. related to the investment. These fees may vary depending on different investment projects and financial instruments.

For example, when purchasing a stock, investors need to pay transaction fees and taxes, etc., which will be included in the purchase cost. Therefore, when calculating the principal, the purchase cost needs to be used as the investment amount, and the transaction fees and taxes need to be calculated as related fees.