Quadratic 6

Algebra Level 3

x 2 3 x + 2 = 0 \large x^2 - 3 | x | + 2 = 0

Find the number of real solutions of the equation above.

4 1 3 2

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4 solutions

Pranjal Jain
Feb 21, 2015

x 2 = x 2 x^2=|x|^2

x = p |x|=p

p 2 3 p + 2 = 0 ( p 2 ) ( p 1 ) = 0 p = 1 , 2 p^2-3p+2=0\\\Rightarrow (p-2)(p-1)=0\\\Rightarrow p=1,2

x = 1 , 2 x = ± 1 , ± 2 |x|=1,2\Rightarrow x=\pm 1,\pm 2

how is 4 the answer?

Niharika Rajanala - 5 months, 3 weeks ago

First, this problem is too overrated :D

Second, we divide this equation into 2 cases:

Case 1: If x 0 x = x x 2 3 x + 2 = 0 x { 1 ; 2 } x \geq 0\Leftrightarrow \left| x \right| =x \Leftrightarrow { x }^{ 2 }-3x+2=0 \Leftrightarrow x\in \{ 1;2\} , which gives us 2 2 solutions of x x

Case 2: If x < 0 x = x x 2 + 3 x + 2 = 0 x { 1 ; 2 } x<0\Leftrightarrow \left| x \right| =-x \Leftrightarrow { x }^{ 2 }+3x+2=0 \Leftrightarrow x\in \{ -1;-2\} , which gives us another 2 2 solutions of x x

In total, there are 2 + 2 = 4 2+2=\boxed{4} solutions of x x satisfy the equations.

Though this is a relatively small matter, shouldn't x=0 be included in some part of the domain, as the function is defined there ?

i.e. Case 1 : x 0 x \geq 0 etc

Case 2 : x < 0 x < 0 ... etc

But aside from that technicality, I did it exactly the same way ^^

Shani P - 6 years, 3 months ago
Smarth Mittal
Feb 20, 2015

Number of reals solutions will be 4 because it is mod x

So there will be two equations

so number of real solutions comes out to be 4

Notice that the function is even: we can restrict our study to x 2 3 x + 2 = ( x 2 ) ( x 1 ) x^2-3x+2=(x-2)\cdot(x-1) which has two positive roots. By simmetry (with respect to the y axis), we can conclude that the number of solutions of the starting equation is therefore 2 2 = 4 2 \cdot2=4 .

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