( x − 3 ) 3 + ( x − 7 ) 3 = ( 2 x − 1 0 ) 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.
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There is a mistake in the 3rd line while writing the identity. Edit it accordingly.
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The solution has been edited since. It was wrong when I posted that comment.
Let y = x − 5 . Then the equation becomes
( y + 2 ) 3 + ( y − 2 ) 3 = ( 2 y ) 3 = 8 y 3 .
Now when we expand the two terms on the LHS we have several terms cancel, leaving us with
⟹ 2 y 3 + 2 ∗ ( 2 3 ) ∗ 4 y = 8 y 3 ⟹ 6 y ( y 2 − 4 ) = 0 ,
which has solutions y = 0 , 2 , − 2 , which sum to 0 .
But since x = y + 5 , the real roots of the original equation will be 3 , 5 and 7 , which sum to 1 5 .
Alternatively, going back to the equation ( y + 2 ) 3 + ( y − 2 ) 3 = 8 y 3 , we can observe that if y = a is a solution then 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 .
This, coupled with the observation that y = 0 is a solution, implies that the sum of the roots to this equation is a + ( − a ) + 0 = 0 , and thus the sum of the roots to the original equation is 3 ∗ 5 = 1 5 .
Let f ( x ) = ( x − 3 ) 3 + ( x − 7 ) 3 − ( 2 x − 1 0 ) 3 .
Then we note that:
f ( 3 ) = 0 + ( − 4 ) 3 − ( − 4 ) 3 = 0 + 6 4 − 6 4 = 0
f ( 5 ) = 2 3 + ( − 2 ) 3 − 0 = 8 − 8 − 0 = 0
f ( 7 ) = 4 3 + 0 − 4 3 = 6 4 + 0 − 6 4 = 0
Since f ( x ) is a polynomial with a degree of 3 , it can only have maximum 3 roots. Therefore, the roots are 3 , 5 and 7 and the sum of roots = 3 + 5 + 7 = 1 5
Let ( x − 3 ) = a , ( x − 7 ) = b and ( 1 0 − 2 x ) = c . Frist we have a + b + c = 0 , so a 3 + b 3 + c 3 = 3 a b c . Therefore a b c = 0 = > x 1 = 3 ; x 2 = 7 ; x 3 = 5
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.
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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Let ( x − 3 ) = a and ( x − 7 ) = b , then ( 2 x − 1 0 ) = a + b
We have the equation a 3 + b 3 = ( a + b ) 3
Exapnding the RHS, a 3 + b 3 = a 3 + b 3 + 3 a b ( a + b )
3 a b ( a + b ) = 0
Solutions are a = 0 , b = 0 and a + b = 0
Therefore x = 3 , x = 7 and x = 5
The sum of real roots is 3 + 5 + 7 = 1 5