What is the relation between Ta and Tm of the primers in PCR reactions? ?

The Tm is the melting temperature. The Ta is the optimum annealing temperature (for use in your annealing step of the PCR reaction). Yes it is usually around 5 degrees below the Tm (although this varies considerably and I would always advise a temperature gradient PCR to determine the actual optimum annealing temperature for your primers).
The A, T, C and G in that formula refers to the number of time the bases are present in your primer sequence and it will give you an approximate theroretical Tm. If you then subtract 5 you will get an approximate theoretical Ta.

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