burhan101
Determine the derivative, all the critical numbers and the intervals of increase and decrease
y=-5x²+20x+2
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burhan101
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\[\huge y'=-10x+20\]
\[\huge 0=-10x+20\]
\[x=2\]
burhan101
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i get :
intervals of increase x<2
intervals of decrease x>2
but the answer key says the opposite
Abhishek619
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when
\[y \prime >0, \]
then increasing.
burhan101
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what's in intervals of increase / decrease tho?
koymoi
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I got the same answer as you @burhan101
intervals of increase x<2
intervals of decrease x>2
Abhishek619
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x>2 it decreases
x<2 it increases.
johnweldon1993
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Right...I as well @koymoi ...I think maybe your answer key is wrong @burhan101
burhan101
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think the answer key's wrong then ?
dan815
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yes
burhan101
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yeah, i thought it was a fairly straight forward question
dan815
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graph it and ull know for sure
dan815
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|dw:1371791477746:dw|
xartaan
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dan815
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|dw:1371791538467:dw|
dan815
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oh oops that shud be + when i multiplied by -5
dan815
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|dw:1371791697708:dw|
dan815
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+22
dan815
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|dw:1371791721412:dw|
genius12
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@burhan101 u don't need calculus to find intervals of increase/decrease. This is a parabola with a negative leading coefficient so it opens down. Using -b/2a we get -b/2a = 2, which is the x-coordinate of the vertex of the parabola. Since it opens down, function is increasing on \(\bf (-\infty,2)\) and decreasing on \(\bf (2,\infty)\)