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The function y= x^4+bx^2+8x+1 has a horizontal tangent and a point of inflection for...

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hellome234 | Salutatorian

Posted May 25, 2012 at 12:57 AM via web

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The function y= x^4+bx^2+8x+1 has a horizontal tangent and a point of inflection for the same value of x. Determine the value of b.

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted May 25, 2012 at 1:54 AM (Answer #1)

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The function y= x^4 + b*x^2 + 8x+1. It has a horizontal tangent and a point of inflection for the same value of x.

The slope of a tangent at any point x is given by the value of y' at that point. As the tangent is horizontal at the required value of x, y' = 0

=> 4x^3 + 2bx + 8 = 0

=> 2x^3 + bx + 4 = 0 ...(1)

Also, the function has a point of inflection for the same value of x, y'' = 0

=> 12x^2 + 2b = 0

=> 6x^2 + b = 0 ...(2)

From (1) and (2) one real root of b is obtained and it is equal to b = -6.

The required value of b = -6.

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