Steel Posted August 5, 2007 Share Posted August 5, 2007 I have an equation 13.8 = 5 lg 1000r / 10 + r. I know this can be written as lg r = -0.2r + 0.76 but I cannot figure out how this is achieved. It's been forever since I've done any maths and honestly I seem to have forgotten a lot about what I learned at school Could someone do a step by step demonstration of how to get from the first equation to the second so I would understand where the 'result' comes from. Thank you! Link to comment Share on other sites More sharing options...
insane_alien Posted August 5, 2007 Share Posted August 5, 2007 okay, i'll start you off. hint: multiply both sides by 10+r Link to comment Share on other sites More sharing options...
Steel Posted August 5, 2007 Author Share Posted August 5, 2007 I'm still rather confused but I did get the correct result. Can you tell me if I've done it correctly or not? 13.8= 5 lg 1000r/10 + r 13.8 = 5 lg 100r + r -5 lg 100r = r - 13.8 lg 100r = -0.2r + 2.76 2 lg r = -0.2r + 2.76 lg r = -0.2r + 2.76 - 2 lg r = -0.2r + 0.76 Link to comment Share on other sites More sharing options...
Dc Nilbog Posted August 6, 2007 Share Posted August 6, 2007 Correct me if I am wrong, but I think what he meant was, well, consider this example: To determine Y> X = Y / 2 X*2 = Y Y = X / 2 I hope that helped Dc Nilbog Link to comment Share on other sites More sharing options...
the tree Posted August 6, 2007 Share Posted August 6, 2007 Correct me if I am wrong, but I think what he meant was, well, consider this example: To determine Y> X = Y / 2 X*2 = Y Y = X / 2 I hope that helped Dc Nilbog WHAT Link to comment Share on other sites More sharing options...
insane_alien Posted August 6, 2007 Share Posted August 6, 2007 Correct me if I am wrong, but I think what he meant was, well, consider this example: To determine Y> X = Y / 2 X*2 = Y Y = X / 2 I hope that helped Dc Nilbog :eek::eek::eek: Link to comment Share on other sites More sharing options...
Dc Nilbog Posted August 7, 2007 Share Posted August 7, 2007 Sorry I was meant to say: To determine Y> X = Y / 2 X*2 = Y Y = X*2 But hey, it's the first step that was important right? Dc Nilbog Link to comment Share on other sites More sharing options...
Country Boy Posted August 7, 2007 Share Posted August 7, 2007 I'm still rather confused but I did get the correct result. Can you tell me if I've done it correctly or not? You posted a question, got one response that said "first multiply both sides by 10+ r" and just ignore it? 13.8= 5 lg 1000r/10 + r Okay, first, do you really mean 13.8= 5 lg (1000r/10)+ r or 13.8= 5 (lg(1000r)/10)+ r or 13.8= 5 lg(1000r)/(10+ r)? insane alien was assuming you meant the last. 13.8 = 5 lg 100r + r But this is true only if you mean the first. -5 lg 100r = r - 13.8lg 100r = -0.2r + 2.76 2 lg r = -0.2r + 2.76 Assuming you meant the first of the three options above, this is correct so far. lg r = -0.2r + 2.76 - 2 No. "2 lg r" is 2 MULTIPLIED by lg r. The "inverse" of that is to DIVIDE by lg r, not subtract it. lg r = -0.2r + 0.76 Link to comment Share on other sites More sharing options...
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