Sum of one row multiplied by another row

Hi, I'm new here and would appreciate some help putting together a formula that I'm sure is quite simple. I need the sum of A1*A2+B1*B3+C1*C3, etc

Thanks, Amanda

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did you ment: A1*A3+B1*B3+C1*C3

If yes then use: =SUMPRODUCT(A1:C1,A3:C3)