If
f ( x ) = x 1 1 − 1 0 x 1 0 + 2 2 x 9 + x 7 − 1 0 x 6 + 2 2 x 5 + x 2 − 1 0 x + 4 1 ,
then what is the value of f ( 3 + 5 ) ?
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Well done.
f ( x ) = x 1 1 − 1 0 x 1 0 + 2 2 x 9 + x 7 − 1 0 x 6 + 2 2 x 5 + x 2 − 1 0 x + 2 2 x + 4 1
Let g ( x ) = x 2 − 1 0 x + 2 2 . then we have:
f ( x ) = x 9 g ( x ) + x 5 g ( x ) + g ( x ) + 1 9 = ( x 9 + x 5 + 1 ) g ( x ) + 1 9
We note that the roots of g ( x ) are 5 ± 3 . Therefore, g ( 3 + 5 ) = 0 .
Therefore, f ( 3 + 5 ) = 0 + 1 9 = 1 9
If you use synthetic division, it turns out to be really easy.
Just factorize the equation given in the question as done in the following line { x }^{ 9 }({ x }^{ 2 }-10x+22)+{ x }^{ 5 }({ x }^{ 2 }-10x+22)+{ x }^{ 2 }-10x+41 now just check the first two brackets ,as they are same so take them as common and solve the equation in the brackets u will find that it has (\sqrt { 3 } \pm 5) as its solutions hence it becomes zero when we find f(\sqrt { 3 } +5 ) and the left equation can be easily solved.
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x − 5 = √ 3 Squaring ⇒ x 2 − 1 0 x + 2 2 = 0 … ( 1 ) The required expression is S = x 1 1 − 1 0 x 1 0 + 2 2 x 9 + x 7 − 1 0 x 6 + 2 2 x 5 + x 2 − 1 0 x + 4 1 = x 9 ( x 2 − 1 0 x + 2 2 ) + x 5 ( x 2 − 1 0 x + 2 2 ) + ( x 2 − 1 0 x + 2 2 ) + 1 9 Now putting the value of ( 1 ) ) ⇒ S = 0 + 0 + 0 + 1 9 = 1 9