I am not sure what you are asking, since I have never heard the term "decrease a formula," but my best guess is this:
Assume you have a formula =SUM(A1:A23) and you want to decrease the sum by 6, you would enter the formula =SUM(A1:A23)-6.
It is a formula that the user creates themselves, instead of using the built-in functions.
The Clipboard.
You can click on it in the Formula Bar near the top of screen, or you can press the F2 key.
It is a function
It is used to copy the formula down or across.
In Excel an equation can be a formula. All formulas in Excel must start with the equals sign. That is what tells Excel it is a formula.
Not entirely, but you can use a mouse to select ranges you include in a formula.
You would first need to know how the fat in milk is calculated and have a formula for it. You would need to know what values are needed to do it. Then you would need to get the values you need and enter them into cells in the spreadsheet. Using these cell references to build your formula, you could do the calculation.
In Excel it allows you to enter formulas using the mouse, by clicking on cells as you type a formula, instead on typing the cell references.
There is no formula error button in Excel. However, there is an Error Checking button on the Formulas ribbon in the Formula Auditing section in Excel 2007.
Under the formulas tab, there is an option called "Show Formulas" in Formula Auditing. This applies for Excel 2010 (what I am using) and should apply for Excel 2003 and 2007 as well. For older options (or if there is no such option in Excel 2003/2007, search for something similar).
The PVIFA formula in excel refers to Present Value Interest Factor of Annuity. This is able to be calculated in an excel document.