The equation ( x − 9 2 ) ( x − 9 3 ) ( x − 9 4 ) … ( x − 9 1 0 ) = 1 has 9 distinct complex solutions x 1 , x 2 , x 3 , … , x 9 . Find the value of x 1 9 + x 2 9 + x 3 9 + … + x 9 9 .
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If we generalise, then the answer for arbitrary n comes out to be 2 n ( n + 5 ) . In simpler terms,
If x 1 , x 2 , x 3 . . . . . x n are roots of the equation ( x − n 2 ) ( x − n 3 ) ( x − n 4 ) . . . . . ( x − n n + 1 ) = 1
Then, x 1 n + x 2 n + x 3 n + . . . . . x n n = 2 n ( n + 5 )
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Relevant wiki: Vieta's Formula Problem Solving - Intermediate
Let the roots of the equation x 9 + m 1 x 8 + m 2 x 7 + m 3 x 6 + m 4 x 5 + m 5 x 4 + m 6 x 3 + m 7 x 2 + m 8 x + m 9 = 0 be a 1 , a 2 , a 3 . . . a 9 .
Then a 1 a 2 a 3 . . . a 9 = − m 9 ,and a 1 9 + a 2 9 + a 3 9 + . . . + a 9 9 = f 9 ( m 1 , m 2 , m 3 , . . . m 8 ) + n m 9 where f 9 is some function,and n is a constant.
Let's find n first.Consider the equations x 9 = 0 and x 9 − 1 = 0 .
The first equation's roots are b 1 , b 2 , b 3 , . . . b 9 ,and the second one's are c 1 , c 2 , c 3 , . . . c 9
According to the above,we have b 1 9 + b 2 9 + b 3 9 + . . . + b 9 9 = 0 = f 9 ( 0 , 0 , 0 , . . . , 0 ) + 0 n and c 1 9 + c 2 9 + c 3 9 + . . . + c 9 9 = 9 = f 9 ( 0 , 0 , 0 , . . . , 0 ) − 1 n
So we get n = − 9
Consider the 2 equations ( x − 9 2 ) ( x − 9 3 ) ( x − 9 4 ) … ( x − 9 1 0 ) − 1 = 0 and ( x − 9 2 ) ( x − 9 3 ) ( x − 9 4 ) … ( x − 9 1 0 ) = 0 Expand them,and they will be in the form of x 9 + p 1 x 8 + p 2 x 7 + p 3 x 6 + p 4 x 5 + p 5 x 4 + p 6 x 3 + p 7 x 2 + p 8 x − 9 1 0 ! − 1 = 0 and x 9 + p 1 x 8 + p 2 x 7 + p 3 x 6 + p 4 x 5 + p 5 x 4 + p 6 x 3 + p 7 x 2 + p 8 x − 9 1 0 ! = 0 where p 1 , p 2 , . . . , p 8 are constants.
According to the above,we have x 1 9 + x 2 9 + x 3 9 + . . . + x 9 9 = f 9 ( p 1 , p 2 , p 3 , . . . , p 8 ) + 9 ( 9 1 0 ! + 1 ) and 2 + 3 + 4 + . . . + 1 0 = 5 4 = f 9 ( p 1 , p 2 , p 3 , . . . , p 8 ) + 9 9 1 0 !
Subtract and get x 1 9 + x 2 9 + x 3 9 + . . . + x 9 9 = 6 3
To get a clearer idea about expressing a 1 9 + a 2 9 + a 3 9 + . . . + a 9 9 in the form of f 9 ( m 1 , m 2 , m 3 , . . . m 8 ) + n m 9 ,here is an example of a third degree one.
If the equation x 3 + p x 2 + q x + r = 0 has roots a , b , c ,then
a 3 + b 3 + c 3 = ( a + b + c ) 3 − 3 ( a + b + c ) ( a b + b c + c a ) + 3 a b c = − p 3 + 3 p q − 3 r If f 3 ( x , y ) = − x 3 + 3 x y , m = − 3 ,then a 3 + b 3 + c 3 can be expressed in the form of f 3 ( p , q ) + m r