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Homework Statement:

Find:
A) vo
B) io
C) power dissipated in 6 ohm resistor
D) power developed by current source
Relevant Equations:
 Current and Voltage divider equations. Ohm's law.
A) Find the equivalent resistance (10 ohms). Find the voltage of the current source (4.2*10 = 42 V). Use a voltage divider to find vo = 42*(20/(10+20))=28V. My question is why do we need to add 10, which is R equivalent to 20 again? I thought I could just put 10 in the denominator, since this is R equivalent.
B) Similar questions. Voltage across 90 ohm resistor = 42*((90 in parallel with 10)/(6+(90 in parallel with 10)) = 25.2 V. Current across 90 ohm resistor = 25.2/90 = 0.28 A.
C) Power dissipated by 6 ohm resistor = (Voltage across current source  voltage across 90 ohm resistor)^2 / 6. My question here is how do I know which is positive and which is negative here in terms of subtraction/addition.