C'mon it's quite easy

Algebra Level 5

The line y = m x + 1 y=mx+1 touches the curve y = x 4 + 2 x 2 + x y=-x^4+2x^2+x at two points P ( x 1 , y 1 ) P(x_1,y_1) and Q ( x 2 , y 2 ) Q(x_2,y_2) . Find the value of ( x 1 2 + x 2 2 + y 1 2 + y 2 2 ) \left( x_1^2+x_2^2+y_1^2+y_2^2\right)


The answer is 6.

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4 solutions

Chew-Seong Cheong
Feb 27, 2015

Since the line y = m x + 1 y = mx +1 touches the curve y = x 4 + 2 x 2 + x y=-x^4+2x^2+x at P ( x 1 , y 1 ) P(x_1, y_1) and Q ( x 2 , y 2 ) Q(x_2, y_2) (see above), at these two points the following equations are satisfied.

{ y = x 4 + 2 x 2 + x = m x + 1 . . . ( 1 ) d y d x = 4 x 3 + 4 x + 1 = m . . . ( 2 ) \begin{cases} y =-x^4+2x^2+x = mx + 1 & ...(1) \\ \dfrac {dy}{dx} = -4x^3+4x+1 = m & ...(2) \end{cases}

E q 1 E q . 2 × x : 3 x 4 2 x 2 = 1 ( 3 x 2 + 1 ) ( x 2 1 ) = 0 Eq1-Eq.2\times x: \quad \Rightarrow 3x^4-2x^2 = 1 \quad \Rightarrow (3x^2+1)(x^2-1)=0

x = ± 1 { x 1 = 1 y 1 = 1 + 2 + 1 = 2 x 2 = 1 y 1 = 1 + 2 1 = 0 \Rightarrow x = \pm 1\Rightarrow \begin{cases} x_1=1 & \Rightarrow y_1 = -1+2+1 = 2 \\ x_2=-1 & \Rightarrow y_1 = -1+2-1 = 0 \end{cases}

x 1 2 + x 2 2 + y 1 2 + y 2 2 = 1 + 1 + 4 + 0 = 6 \Rightarrow x_1^2 + x_2^2+y_1^2 + y_2^2 = 1+1+4+0 = \boxed{6}

same method....

Shanthan Kumar - 6 years, 3 months ago

Great solution!!

Akshat Sharda - 5 years, 8 months ago

Good Method

Harsh Sharma - 5 years, 8 months ago
Sandeep Bhardwaj
Feb 26, 2015

According to the question we can say that :

x 4 + 2 x 2 + x = m x + 1 -x^4+2x^2+x=mx+1 has solutions x 1 , x 1 x_1,x_1 and x 2 , x 2 x_2,x_2 .

x 4 2 x 2 + ( m 1 ) x + 1 = ( x x 1 ) 2 ( x x 2 ) 2 \implies x^4-2x^2+(m-1)x+1=(x-x_1)^2 \cdot (x-x_2)^2

On comparing coefficients, we get : x 1 = 1 , x 2 = 1 x_1=1,x_2=-1 and m = 1 m=1

Putting values of x 1 , x 2 x_1,x_2 in the equation of curve, we get y 1 = 2 , y 2 = 0 y_1=2,y_2=0 .

Hence x 1 2 + x 2 2 + y 1 2 + y 2 2 = 6 \boxed{x_1^2+x_2^2+y_1^2+y_2^2=6} .

enjoy !

Same method sir , +1!

Also sir , I think there's a small typo in the 2 n d 2^{nd} line of your solution , y 1 , y 2 y_{1} , y_{2} are missing and instead you have mentioned the x coordinates .

A Former Brilliant Member - 6 years, 3 months ago

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That's not a typo, It will be the same as written x 1 , x 1 , x 2 , x 2 x_1,x_1,x_2,x_2 means x 1 x_1 is two-times repeated root and same with x 2 x_2 .

Sandeep Bhardwaj - 6 years, 3 months ago

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Oh !! I have understood it now sir. I had read it in haste, sorry for troubling you .

A Former Brilliant Member - 6 years, 3 months ago

Well I did a bit differently . I equated the slopes to get a cubic and the equations to get a quartic. The two must have two common solutions. Hence the answer

Rohit Shah - 6 years, 3 months ago
Rohit Shah
Feb 26, 2015

The point of intersection of two curves is given by equating the line and curve. So , x 4 2 x 2 + ( m 1 ) x + 1 = 0 { x }^{ 4 }-2{ x }^{ 2 }+(m-1)x+1=0 At these points of intersection the slopes of the two curves are equal as well, So 4 x 3 4 x + m 1 = 0 4{ x }^{ 3 }-4x+m-1=0 The two equations must have two common solutions. x = + 1 , 1 x=+1,-1 satisfies them hence the answer

m=1 ; x= 1,-1

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