anonymous
  • anonymous
Find the values of a and b if the function y= 2x^3+ax^2+bx+36 has a local maximum when x=-4 and a local minimum of x=5
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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anonymous
  • anonymous
I know y' = 6x^2 + 2ax +b but then I was keep subbing in x=-4 only then what is the x=5 for?
anonymous
  • anonymous
these are the x values when y' = 0. you'll need both of them to find a and b.
anonymous
  • anonymous
yes I put y' as 0, and then sub in x=4 but i got b= -30 and a = 45/4 my friend got it right, but he uses x=5 as well..but i dont know why

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anonymous
  • anonymous
so, |dw:1332380629364:dw|
anonymous
  • anonymous
why do u need to use both x=-4 and x=5?
anonymous
  • anonymous
you actually have to use both of these equations because you're looking for two unknowns, a and b. you're actually solving a system, 2 equations with 2 unknowns....
anonymous
  • anonymous
so if it is solving for 3 unknowns, then need to use 3 equations..?
anonymous
  • anonymous
and do i juz add both equations together after?
anonymous
  • anonymous
|dw:1332380897982:dw|
anonymous
  • anonymous
if that's the way you solve your system. I set my equations like that because I usually do elimination...
anonymous
  • anonymous
ohh, so u can never get answers of 2 unknowns with using only 1 equation?
anonymous
  • anonymous
no...
anonymous
  • anonymous
cuz what I do is find that a = 96+b/ 8 and then sub it back to the equation and find b
anonymous
  • anonymous
yes.
anonymous
  • anonymous
ohh ok :DDD thanks again !
anonymous
  • anonymous
np

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