Khi bạn nhập công thức, bạn đôi khi có thể nhìn thấy một tin nhắn từ Excel như một trong những thể hiện trong Hình 2-7. Điều này chỉ ra rằng công thức mà bạn chỉ cần nhập vào sẽ cho kết quả trong một tài liệu tham khảo vòng tròn | Chapter 2 Basic Facts about Formulas 51 Dealing with Circular References When you enter formulas you may occasionally see a message from Excel like the one shown in Figure 2-7. This indicates that the formula you just entered will result in a circular reference. A circular reference occurs when a formula refers to its own value either directly or indirectly. For example if you enter A1 A2 A3 into cell A3 this produces a circular reference because the formula in cell A3 refers to cell A3. Every time the formula in A3 is calculated it must be calculated again because A3 has changed. The calculation would go on forever. In other words the answer never gets resolved. Figure 2-7 Excel s way of telling you that your formula contains a circular reference When you enter a formula that contains a circular reference Excel displays a dialog box with three options Click OK to attempt to locate the circular reference. Click Cancel to enter the formula as is. Click Help to read more about circular references in the online help. Normally you ll want to correct any circular references so you should choose OK. When you do so Excel displays its Circular Reference toolbar see Figure 2-8 . On the Circular Reference toolbar click the first cell in the Navigate Circular Reference drop-down list box and then examine the cell s formula. If you cannot determine whether the cell is the cause of the circular reference click the next cell in the Navigate Circular Reference drop-down list box. Continue to review the formulas until the status bar no longer displays Circular. 52 Part 1 Basic Information Figure 2-8 The Circular Reference toolbar There are a few situations in which you may want to use a circular reference intentionally. Refer to Chapter 16 for some examples. If you ignore the circular reference message by clicking Cancel Excel enables you to enter the formula and displays a message in the status bar reminding you that a circular reference exists. In this case the message reads .
