Quick Algebra Identities Repeated

Algebra Level 2

2 x 1 + 2 x + 1 2 x + 1 2 x 1 = 5 3 , x = ? \large \dfrac{\color{#D61F06}{ \sqrt{2x-1} }+\color{#20A900}{\sqrt{2x+1}} }{\color{#20A900}{ \sqrt{2x+1}}-\color{#D61F06}{\sqrt{2x-1}} }=\color{#3D99F6}{\dfrac{5}{3}} \quad, \quad \color{magenta}{x} = \ ?

2 3 \dfrac{2}{3} 5 7 \dfrac{5}{7} 1 2 \dfrac{1}{2} 17 30 \dfrac{17}{30} 19 25 \dfrac{19}{25}

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4 solutions

Akhil Bansal
Nov 29, 2015

2 x 1 + 2 x + 1 2 x + 1 2 x 1 = 5 3 \large\Rightarrow \dfrac{ \sqrt{2x-1} +\sqrt{2x+1} }{ \sqrt{2x+1} - \sqrt{2x-1} } = \dfrac{5}{3} Applying Componento and Dividendo , ( 2 x 1 + 2 x + 1 ) + ( 2 x + 1 2 x 1 ) ( 2 x 1 + 2 x + 1 ) ( 2 x + 1 2 x 1 ) = 5 + 3 5 3 \large \Rightarrow\dfrac{ (\sqrt{2x-1} +\sqrt{2x+1} ) +(\sqrt{2x+1} - \sqrt{2x-1})}{(\sqrt{2x-1} +\sqrt{2x+1}) - (\sqrt{2x+1} - \sqrt{2x-1} )} = \dfrac{5+3}{5-3} This simplifies to

2 x + 1 2 x 1 = 4 \large\Rightarrow \dfrac{\sqrt{2x+1}}{\sqrt{2x-1}}=4 Squaring both sides and cross multiplying the terms, 2 x + 1 + 16 ( 2 x 1 ) \large\Rightarrow 2x+1+16(2x-1) x = 17 30 \large\Rightarrow x=\dfrac{17}{30}

Moderator note:

Great solution! Componendo does make it easier to manipulate such expressions.

Exactly the same way man!!!

Noel Lo - 5 years, 6 months ago

I used an almost similar method

Shreyash Rai - 5 years, 6 months ago

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Which one?

Akhil Bansal - 5 years, 6 months ago

How did you then do it?

Ananya Prakash - 5 years, 6 months ago

What I did is : 3√(2x-1)+3√(2x+1)=5√(2x+1)-5√(2x-1) then Then 8√(2x-1)=2√(2x+1) Then 4√(2x-1)=√(2x+1) Then (4√(2x-1))^2=(√(2x+1))^2 Then 16(2x-1)=2x+1 Then 32x-1=2x+1 Then 30x=17 Then x=17/30

Omar Mohd Qaisieh - 5 years, 6 months ago

Even though the answer is right but the way complety wrong. For instance 2/4=1/2 ,but that absolutely does not mean that 4=2. So if a/b=c/d then (a) will not necessarily equal (c) ,and (b)will not necessarily equal (d).

Omar Mohd Qaisieh - 5 years, 6 months ago

Cross multiplying works too.

Brian Wang - 5 years, 6 months ago

2 x 1 + 2 x + 1 2 x + 1 2 x 1 = 5 3 \Rightarrow\dfrac{ \sqrt{2x-1} +\sqrt{2x+1} }{ \sqrt{2x+1} - \sqrt{2x-1} } = \dfrac{5}{3}
Let 2 x + 1 = a \sqrt{2x+1}=a and 2 x 1 = b \sqrt{2x-1}=b .
b + a a b = 5 3 \dfrac{b+a}{a-b}=\dfrac{5}{3}
Multiplying by ( a + b ) (a+b) on numerator and denominator.
b + a a b × a + b a + b = 5 3 \dfrac{b+a}{a-b}×\dfrac{a+b}{a+b}=\dfrac{5}{3}
a 2 + b 2 + 2 a b a 2 b 2 = 5 3 \dfrac{a^2+b^2+2ab}{a^2-b^2}=\dfrac{5}{3}
2 x + 4 x 2 1 1 = 5 3 \dfrac{2x+\sqrt{4x^2-1}}{1}=\dfrac{5}{3}
Now cross multiplying.
6 x + 3 4 x 2 1 = 5 6x+3\sqrt{4x^2-1}=5
6 x 5 = 3 4 x 2 1 6x-5=-3\sqrt{4x^2-1}
Squring both sides.
36 x 2 + 25 60 x = 36 x 2 9 36x^2+25-60x=36x^2-9
60 x = 34 -60x=-34
x = 17 30 x=\boxed{\dfrac{17}{30}}


yes this is exactly how i did it

Shreyash Rai - 5 years, 6 months ago

Componendo and Dividendo is a lot better.

Ananya Prakash - 5 years, 6 months ago

This is my way too :)

Zhi Yang Marcus - 5 years, 6 months ago

the same way i did

Shreyansh Choudhary - 5 years, 6 months ago
Julia Scylla
Dec 2, 2015

Let 2 x 1 = A \sqrt { 2x-1 } =A Let 2 x + 1 = B \sqrt { 2x+1 } =B

A + B B A = 5 3 \frac { A+B }{ B-A } =\frac { 5 }{ 3 } 3 A + 3 B = 5 B 5 A 3A+3B=5B-5A 8 A = 2 B 8A=2B 4 A = B 4A=B ( 4 A ) 2 = ( B ) 2 (4A)^{ 2 }=(B)^{ 2 } 32 x 16 = 2 x + 1 32x-16=2x+1 30 x = 17 30x=17 x = 17 30 x=\frac { 17 }{ 30 }

William Shao
Nov 30, 2015

Hmmm... All the previous solutions look more complicated (I have no idea what Componento and Dividendo is lol). Here is my solution: a + b b a = 5 3 \Rightarrow \frac { a+b }{ b-a } =\frac { 5 }{ 3 } 5 b 5 a = 3 a + 3 b \Rightarrow 5b-5a=3a+3b 2 b = 8 a b = 4 a \Rightarrow 2b=8a\Rightarrow b=4a 2 x + 1 = 4 2 x 1 \Rightarrow \sqrt { 2x+1 } =4\sqrt { 2x-1 } Square both sides to yield: 2 x + 1 = 32 x 16 \Rightarrow 2x+1=32x-16 And so 17 = 30 x \Rightarrow 17=30x x = 17 30 \Rightarrow x=\frac { 17 }{ 30 }

I think that we must follow the method of componendo and dividendo because this was the way it was supposed to be done ( since, the problem maker strategically gave 5/3). Also, componendo and dividendo is very helpful sometimes, and can significantly simply the solution if done using algrbra....

It states that if- A/B=C/D, then, A+B/A-B=C+D/C-D

Pranjal Mittal - 5 years, 6 months ago

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