Simplify rt= 1/[(1/r1)+ (1/r2)]
That looks like the formula for the "total" resistance, rt, of two resistors in parallel, r1 and r2. The t, 1 and 2 are subscripts.
It cannot be made much simpler than that, but it can be rewritten
1/rt = 1/r1 + 1/r2
which is a bit more compact and easier to remember.
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