Understanding Two Coils - V1/V2 & I1/I2 Ratios

[nhrock3]

http://i47.tinypic.com/k0n0gi.jpg
i know that V1/V2=1/a
I1/I2=-a

how they interpret the left resistor
to be like this ?

 

[vk6kro]

The impedance ratio of a transformer is equal to the square of the turns ratio.

So, in this case, the turns ratio is equal to 100 so the impedance ratio is 100 * 100 or 10000.

Ra was there before, so it is unchanged, but Rb is seen from the other side of the transformer as being increased by a factor of 10000.