Genady Posted November 3, 2024 Posted November 3, 2024 It is not a difficult equation. I wonder, how people approach it. Here it is: \[x+\frac{1}{x}=4\frac{1}{4}\] (Please, use spoiler in your response.)
joigus Posted November 3, 2024 Posted November 3, 2024 Spoiler Method 1: 4 and 1/4 can be spotted directly, as 4+1/4 = 1/4+1/(1/4), and it's clearly equivalent to a quadratic equation (once zero is ruled out as a solution), so those are the only roots. Method 2: I rule out x=0, as it is not a solution. Then I mulltiply by x to obtain the quadratic eq. 4x2-17x+4=0, which gives x=4 and x=1/4 via the algorithm for solving quadratic equations. Spotting obvious solutions first can sometimes be very helpful to obtain the non-obvious ones when the equation is polynomial and has degree > 4. Example: x5=x What would you do? 1
Genady Posted November 3, 2024 Author Posted November 3, 2024 (edited) Spoiler First, x=0. Then, divide by x to get x4=1 and thus x=1, -1, i, -i. Edited November 3, 2024 by Genady 1
sethoflagos Posted November 3, 2024 Posted November 3, 2024 Spoiler x=4 is one obvious root just on inspection, but if we ignore that: Multiply both sides by 4x to give 4x2 + 4 = 17x or 4x2 - 17x+ 4 = 0 By formula x =(17 +/- (172 - 43)1/2)/8 Ans x = 4 or 1/4 Ooh! That's neat! ... what else could you be looking for? 1
Genady Posted November 3, 2024 Author Posted November 3, 2024 (edited) 11 minutes ago, sethoflagos said: Reveal hidden contents x=4 is one obvious root just on inspection, but if we ignore that: Multiply both sides by 4x to give 4x2 + 4 = 17x or 4x2 - 17x+ 4 = 0 By formula x =(17 +/- (172 - 43)1/2)/8 Ans x = 4 or 1/4 Ooh! That's neat! ... what else could you be looking for? Spoiler Many go straight to 17/4 and miss the first approach. OTOH, some see both answers by the first approach. Edited November 3, 2024 by Genady
TheVat Posted November 3, 2024 Posted November 3, 2024 Spoiler For me, one solution popped out by seeing that 4 1/4 can be said as "4 and one fourth." So then x + 1/x is obviously 4. Then say the right hand part backwards, "one fourth and four" and you get the other solution, 1/4. It's really just noticing the RH part is just a number and its reciprocal. 31 minutes ago, sethoflagos said: what else could you be looking for? You did it the hard way. 🙂 1
sethoflagos Posted November 3, 2024 Posted November 3, 2024 4 minutes ago, TheVat said: Hide contents You did it the hard way. 🙂 Force of habit. I ALWAYS do the check longhand just to be sure. (Chem Eng thing) 1
Genady Posted November 3, 2024 Author Posted November 3, 2024 5 hours ago, sethoflagos said: Reveal hidden contents x=4 is one obvious root just on inspection, but if we ignore that: Multiply both sides by 4x to give 4x2 + 4 = 17x or 4x2 - 17x+ 4 = 0 By formula x =(17 +/- (172 - 43)1/2)/8 Ans x = 4 or 1/4 Ooh! That's neat! ... what else could you be looking for? I just thought of something else, for a case when one sees one solution but not the other: Spoiler Instead of solving the quadratic equation, one can notice that after multiplying by x the quadratic equation will have a constant term 1, which means that the product of the roots is 1, which gives the second root, 1/4.
sethoflagos Posted November 4, 2024 Posted November 4, 2024 2 hours ago, Genady said: I just thought of something else, for a case when one sees one solution but not the other: Reveal hidden contents Instead of solving the quadratic equation, one can notice that after multiplying by x the quadratic equation will have a constant term 1, which means that the product of the roots is 1, which gives the second root, 1/4. My impression of equations of that type are dominated by: x - 1/x = 1
Genady Posted November 4, 2024 Author Posted November 4, 2024 12 minutes ago, sethoflagos said: My impression of equations of that type are dominated by: x - 1/x = 1 Could you elabotate?
sethoflagos Posted November 4, 2024 Posted November 4, 2024 (edited) 9 hours ago, Genady said: Could you elabotate? Spoiler Your OP equation is one of a family whose roots are the positive inverse of each other. Conversely mine is one of a family whose roots have a negative inverse relationship. eg. x - 1/x = 3 3/4 solves to x = 4, -1/4 x - 1/x = 1 solves to the golden ratio (as I'm sure you know) and its negative inverse. A deep, deep rabbit hole I've ventured into on occasion. Compare and contrast: x + 1/x = 51/2 Edited November 4, 2024 by sethoflagos 1
Genady Posted November 4, 2024 Author Posted November 4, 2024 (edited) 1 hour ago, sethoflagos said: Reveal hidden contents Your OP equation is one of a family whose roots are the positive inverse of each other. Conversely mine is one of a family whose roots have a negative inverse relationship. eg. x - 1/x = 3 3/4 solves to x = 4, -1/4 x - 1/x = 1 solves to the golden ratio (as I'm sure you know) and its negative inverse. A deep, deep rabbit hole I've ventured into on occasion. Compare and contrast: x + 1/x = 51/2 I see. A general form Spoiler \[x+\frac{a}{x}=b\] has roots with the product \(a\) and the sum \(b\). (However, the OP was about how people approach that equation rather than a mathematics behind it.) (I see how YOU approach it 🙂 ) Edited November 4, 2024 by Genady
Genady Posted November 4, 2024 Author Posted November 4, 2024 I'll post my approach, for the record. Spoiler I wrote the RHS as \[x+\frac{1}{x}=4+\frac{1}{4}\] This immediately gave me one answer. The second answer took a bit more effort, but I don't recall the exact thinking there.
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