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I am struggling to solve this problem and my available time conflicts with my professors available time. Could any of you offer me some help, perhaps describe the method I would use to solve such a problem in a way that I can learn from it?

Thank you. (Problem follows):

Voltage Divider

The circuit designer wants to change the values of R_1 and R_2 so that the design output voltage v_o = 5 V is achieved when the load resistance is R_L = 200 kΩ rather than at no-load. The actual output voltage must not drop below 4.5 V when R_L = 100 kΩ . What is the smallest resistor value that can be used for R_1?

(R_1 and R_2 are unknown quantities.)

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R2 and RL are in parallel so the voltage across them will be the same so can be treated as a single resistor when thinking about the voltage across them.

So

Req(lower) = (1/R2 + 1/100kohms)^-1

Req(upper) = (1/R2 + 1/200kohms)^-1

Vs/(R1 + Req) = I

Hopefully this gets your started, you're looking to produce a system of 2 equations that you can use to solve for R1 and R2

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