Just put the value of x (sounds easy)

Algebra Level 4

If

f ( x ) = x 11 10 x 10 + 22 x 9 + x 7 10 x 6 + 22 x 5 + x 2 10 x + 41 , f(x)=x^{11}-10x^{10}+22x^9+x^7-10x^6+22x^5+x^2-10x+41,

then what is the value of f ( 3 + 5 ) ? f(\sqrt{3}+5)?


The answer is 19.

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4 solutions

Sanjeet Raria
Nov 2, 2014

x 5 = 3 x-5=√3 Squaring x 2 10 x + 22 = 0 ( 1 ) \Rightarrow x^2-10x+22=0…(1) The required expression is S = x 11 10 x 10 + 22 x 9 + x 7 10 x 6 + 22 x 5 + x 2 10 x + 41 S=x^{11}-10x^{10}+22x^9+x^7-10x^6+22x^5+x^2-10x+41 = x 9 ( x 2 10 x + 22 ) + x 5 ( x 2 10 x + 22 ) + ( x 2 10 x + 22 ) + 19 =x^{9}(x^2-10x+22)+x^5(x^2-10x+22)+(x^2-10x+22)+19 Now putting the value of ( 1 ) ) (1)) S = 0 + 0 + 0 + 19 = 19 \Rightarrow S=0+0+0+19=\boxed{19}

Well done.

Ayush Verma - 6 years, 7 months ago

f ( x ) = x 11 10 x 10 + 22 x 9 + x 7 10 x 6 + 22 x 5 + x 2 10 x + 22 x + 41 f(x) = x^{11} - 10x^{10} + 22x^9 + x^7 - 10x^6 + 22x^5 + x^2 - 10x + 22x + 41

Let g ( x ) = x 2 10 x + 22 g(x) = x^2 - 10x +22 . then we have:

f ( x ) = x 9 g ( x ) + x 5 g ( x ) + g ( x ) + 19 = ( x 9 + x 5 + 1 ) g ( x ) + 19 f(x) = x^9g(x) + x^5g(x) + g(x) + 19 = (x^9+x^5+1)g(x) + 19

We note that the roots of g ( x ) g(x) are 5 ± 3 5 \pm \sqrt{3} . Therefore, g ( 3 + 5 ) = 0 g(\sqrt{3}+5) = 0 .

Therefore, f ( 3 + 5 ) = 0 + 19 = 19 f(\sqrt{3}+5) = 0 + 19 = \boxed{19}

James Wilson
Dec 25, 2017

If you use synthetic division, it turns out to be really easy.

Sagar Verma
Nov 2, 2014

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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