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+//chapter9
+//example9.20
+//page170
+
+R=5 // kilo ohm
+Rl=10 // kilo ohm
+Ei=120 // V
+Vz=50 // V
+
+V=Ei*Rl/(R+Rl) // voltage across open circuit if zener diode is removed
+Vo=Vz // output voltage
+V_R=Ei-Vz // drop across R
+Il=Vz/Rl // load current 
+I=V_R/R // current through R
+
+// by Kirchoff first law I=Iz+Il
+Iz=I-Il
+
+printf("output voltage = %.3f V \n",Vo)
+printf("voltage drop across series resistance = %.3f V \n",V_R)
+printf("current through Zener diode = %.3f mA \n",Iz)