How to write a formula in EXCEL?

Question

excel

If the value in cell A3 is greater than 2.5, then +50% must be added to the value in cell I3. How to write this as a function?

Answer 1

=IF(A3>2.5;I3*1.5;"а тут вы не сказали, что делать, если значение не больше")

In Russian Excel, instead of IF it will be IF, in Russian locale the decimal separator will be a comma, probably.
PS Your title is incorrect. This is not "add a function to Excel", which is VBA programming to create custom functions, but simply entering a formula that conditions the data.
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ie:
IF A3>2.5 THEN multiply E3 by G3, and add 50%; IF NO, then simply multiply E3 by G3.