What 2 resistors will give a total resistance of 780 Ω when wired in series and 105 Ω when wired in parallel?
What 2 resistors will give a total resistance of 780 Ω when wired in series and 105 Ω when wired in parallel?
R1 + R2 = 780
1/R1 + 1/R2 = 1/105
how did they get 1/780-R2 + 1/R2 = 1/105
780 / R2(780-R2) = 1/105
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The previous answer is correct, but the "two solutions" are simply the same two resistance values in different order. Since the order of the resistors does not matter, there really is just a single solution here. The resistors are x and (780-x), then
x(780-x)/780 = 105
x^2 - 780x + 81900 = 0
x = 30( 13 +/- sqrt(78 ) = 125.047 and 654.953
Is has two answers.......
use this formula
R2 = [-b+ - (b^2 - 4ac)^0.5] /2a