Horizontal tangent/point of inflection problem

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SUMMARY

The function y = x4 + bx2 + 8x + 1 has a horizontal tangent and a point of inflection at the same x-value. To find the value of b, first calculate the first derivative and set it equal to zero to identify critical points. Next, compute the second derivative, set it to zero, and solve for b. Substitute this value of b back into the first derivative equation to determine the corresponding x-value.

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Homework Statement



The function y=x(to the forth)+bx(squared)+8x+1 has a horizontal tangent and a point of inflection for the same value of x. What must be the value of b?

Homework Equations



I think you have to find the derivative, set it equal to 0 and solve for x

The Attempt at a Solution

 
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That's one thing you have to do alright. But the second derivative also has to be zero, doesn't it? I would suggest you solve the equation for the second derivative equal to zero for b and substitute it into the first equation. THEN solve for x.
 

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