A simple observation would do!

Algebra Level 3

( x 3 ) 3 + ( x 7 ) 3 = ( 2 x 10 ) 3 \large { (x-3) }^{ 3 }+{ (x-7 })^{ 3 }={ (2x-10) }^{ 3 }

Find the sum of all real roots of the equation above.

Note: There may multiple approaches to solve the problem. Thus, the approach you adopt to solve the equation matters more.


The answer is 15.

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6 solutions

Pranshu Gaba
Nov 11, 2014

Let ( x 3 ) = a (x-3) = a and ( x 7 ) = b (x-7) = b , then ( 2 x 10 ) = a + b (2x-10) = a + b

We have the equation a 3 + b 3 = ( a + b ) 3 a^3 + b^3 = (a+b)^3

Exapnding the RHS, a 3 + b 3 = a 3 + b 3 + 3 a b ( a + b ) a^3 + b^3 = a^3 + b^3 + 3ab(a+b)

3 a b ( a + b ) = 0 3ab(a+b) = 0

Solutions are a = 0 , b = 0 a= 0, b=0 and a + b = 0 a + b = 0

Therefore x = 3 , x = 7 x = 3, x = 7 and x = 5 x = 5

The sum of real roots is 3 + 5 + 7 = 15 3+ 5 + 7 = \boxed{15}

There is a mistake in the 3rd line while writing the identity. Edit it accordingly.

Prasun Biswas - 6 years, 6 months ago

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Nothing wrong

Department 8 - 5 years, 11 months ago

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The solution has been edited since. It was wrong when I posted that comment.

Prasun Biswas - 5 years, 11 months ago

Let y = x 5 y = x - 5 . Then the equation becomes

( y + 2 ) 3 + ( y 2 ) 3 = ( 2 y ) 3 = 8 y 3 (y + 2)^{3} + (y - 2)^{3} = (2y)^{3} = 8y^{3} .

Now when we expand the two terms on the LHS we have several terms cancel, leaving us with

2 y 3 + 2 ( 3 2 ) 4 y = 8 y 3 6 y ( y 2 4 ) = 0 \Longrightarrow 2y^{3} + 2*\binom{3}{2}*4y = 8y^{3} \Longrightarrow 6y(y^{2} - 4) = 0 ,

which has solutions y = 0 , 2 , 2 y = 0, 2, -2 , which sum to 0 0 .

But since x = y + 5 x = y + 5 , the real roots of the original equation will be 3 , 5 3, 5 and 7 7 , which sum to 15 \boxed{15} .

Alternatively, going back to the equation ( y + 2 ) 3 + ( y 2 ) 3 = 8 y 3 (y + 2)^{3} + (y - 2)^{3} = 8y^{3} , we can observe that if y = a y = a is a solution then y = a y = -a is also a solution, since

( a + 2 ) 3 + ( a 2 ) 3 = ( a 2 ) 3 ( a + 2 ) 3 = ( ( a + 2 ) 3 + ( a 2 ) 3 ) = 8 a 3 = 8 ( a ) 3 (-a + 2)^{3} + (-a - 2)^{3} = -(a - 2)^{3} - (a + 2)^{3} = -((a + 2)^{3} + (a - 2)^{3}) = -8a^{3} = 8*(-a)^{3} .

This, coupled with the observation that y = 0 y = 0 is a solution, implies that the sum of the roots to this equation is a + ( a ) + 0 = 0 a + (-a) + 0 = 0 , and thus the sum of the roots to the original equation is 3 5 = 15 3*5 = \boxed{15} .

Chew-Seong Cheong
Nov 11, 2014

Let f ( x ) = ( x 3 ) 3 + ( x 7 ) 3 ( 2 x 10 ) 3 f(x) = (x-3)^3 + (x-7)^3 - (2x-10)^3 .

Then we note that:

f ( 3 ) = 0 + ( 4 ) 3 ( 4 ) 3 = 0 + 64 64 = 0 f(3) = 0 + (-4)^3 - (-4)^3 = 0 +64-64 = 0

f ( 5 ) = 2 3 + ( 2 ) 3 0 = 8 8 0 = 0 f(5) = 2^3 + (-2)^3 - 0 = 8-8-0 = 0

f ( 7 ) = 4 3 + 0 4 3 = 64 + 0 64 = 0 f(7) = 4^3+0-4^3 = 64+0-64 = 0

Since f ( x ) f(x) is a polynomial with a degree of 3 3 , it can only have maximum 3 3 roots. Therefore, the roots are 3 3 , 5 5 and 7 7 and the sum of roots = 3 + 5 + 7 = 15 = 3+5+7 = \boxed{15}

Let ( x 3 ) = a (x-3)=a , ( x 7 ) = b (x-7)=b and ( 10 2 x ) = c (10-2x)=c . Frist we have a + b + c = 0 a+b+c=0 , so a 3 + b 3 + c 3 = 3 a b c a^{3}+b^{3}+c^{3}=3abc . Therefore a b c = 0 = > x 1 = 3 ; x 2 = 7 ; x 3 = 5 abc=0 => x_{1}=3; x_{2}=7; x_{3}=5

Kapil Chandak
Nov 12, 2014

Let(x-3)=a,(x-7)=b then (2x-10)=a+b or -c. So,transferring 2x-10 to L.H.S. we get, a^3 +b^3+(-a-b)^3=0.Here,a+b+c=a+b-a-b=0. So,a^3+b^3+c^3=3 a.b.c, as a+b+c=0 Therefore 0=3.(x-3)(x-7).(2x-10) So, x=3,7,5.Hence,sum of real roots is 15.

Parveen Soni
Nov 10, 2014

Sum of roots=-b/a To find b and a find coefficient of x^2 and x^3 respectively. So value of a=6 and value of b=-90 hence sum of roots=- (-90)/6=15

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