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Stuck on this physics as level question - resistivity

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    Three resistors, of resistance R1, R2 and R3 respectively, are connected in parallel across acell. The cell has negligible internal resistance and electromotive force
    Use the principle of conservation of charge to show that the total resistance R^t of thethree resistors can be found using the expression

    1/Rt = 1/R1 + 1/R2 + 1/R3

    Conservation of charge, Q, gives that

    Qt = Q1 + Q2 + Q2

    or equivalently

    It = I1 + I2 + I3

    where I is the current, I = Q / t.

    As the resistors are connected in parallel, they all have the same voltage V. So using Ohm's law, V = IR, gives

    V/Rt = V/R1 + V/R2 + V/R3.

    Divide by V to get the result.
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