Explaining the positive and negative regions of the output waveform in a circuit involving a Zener diode
During (+) cycle the Zener diode is reverse biased (Vz = 5). The other 5 V will be divided on the two resistors so the voltage across R3 is around 2.5 V.
During the (-) cycle the Zener diode is forward biased (Vf = 0.3 ~ 0.7). Assuming Vf = 0.6 V so -9.4 V will be divided on the resistors, thus the voltage across R3 will be around -4.7 V.