Question 695678: find b if you happen to know the maximum y-value of the function y= -4x^2 + bx -1 is 21.
Answer by stanbon(75887)   (Show Source):
You can put this solution on YOUR website! find b if you happen to know the maximum y-value of the function
y= -4x^2 + bx -1 is 21
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Comment:
Maximum occurs where x = -b/(2a) = -b/(-8) = b/8
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Max = f(b/8) = -4(b/8)^2 + b(b/8) -1 = 21
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-4(b^2/64) + b^2/8 = 22
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-b^2/16 +2(b^2/16) = 22
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-b^2 + 2b^2 = 16*22
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b^2 = 16*22
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b = 4*sqrt(22)
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Cheers,
Stan H.
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