Dragging formulas

Ok so i want to drag a formula however i want one of the values of the formula to change and the other not to change. for example:

c1=a1+b1
c2=a1+b2
c3= a1+b3
: : :

how can i make it do this when i drag? a1 is not just a constant and it depends on other parameters.

please help

Try either

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or

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The dollar to the left of the A makes the column reference absolute and the dollar sign on the left of the row makes the row an absolute.

See this link for help on understanding absolute and relative cell references