tbrooks3
Find the minimum value for the function: y = x^2 + 9x + 18
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samjordon
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do u knwo how to complete the square? or do u factor it?
samjordon
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lets factor its easy
tbrooks3
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its under my quadratic function lesson so idk
samjordon
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what 2 numbers when you multiply them =18 and when u add them =9?
tbrooks3
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is there a number for that?
samjordon
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6*3=18
6+3=9
tbrooks3
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ohhh
tbrooks3
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so is the answer 9
?
samjordon
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yaaaa factoring i just taught this to my class today.
hmmmm nooooo we r factoring. We r trying to figure out 2 numbers that when u mulyiply them give us 18 and when u add them gives us 9
anonymous
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finding the zeros is a long and complicated way to find the minimum
samjordon
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umm well we can complete the square
samjordon
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but i dont think she knows that
Zarkon
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use the vertex formula
anonymous
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minimum is at the vertex, and first coordinate of the vertex of
\[y=ax^2+bx+c\] is always \(-\frac{b}{2a}\)
tbrooks3
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when doing the vertx thing... is the second number the minimum?
anonymous
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in your case it is \(-\frac{9}{2}\) replace \(x\) by \(-\frac{9}{2}\) to find the minimum
anonymous
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minimum means minimum value of \(y\) so it is the second coordinate, not the first
anonymous
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so yes, what you said, the second number is the minimum.
samjordon
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ohhh i like this method. We never learnt this stuff. Like the textbook always made us complete the square. oh cooll
tbrooks3
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can i just plug it into my graphing calculator to get the anwser ?
anonymous
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completing the square is for suckers, unless of course you are solving a quadratic equation
lgbasallote
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last i heard minimum value was in a calculus class o.O i never knew it was in algebra too
Lukecrayonz
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It's in Algebra 2.
Lukecrayonz
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You can use a calculator to solve it
samjordon
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hahah its in math 11 here in canada
anonymous
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\[(-\frac{9}{2})^2+9\times (\frac{-9}{2})+18\] then a calculator if you need one
anonymous
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you turn math up to eleven in canada? mine only goes up to ten
Lukecrayonz
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Ti-84, input the equation into Y= then click 2nd Trace, and do minimum. It will say "Left bound?", go to the left side of the graph and press enter. Right bound? Obviously, right side. It will say "Guess?", which means put it to where you think the minimum y value is.
Lukecrayonz
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So for this, you get x=-4.5 and y=-2.25
tbrooks3
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sooo many numbers ... ahhhhh!!! >_< what about TI-83 plus?
samjordon
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hahaha grade 11 math lol
lgbasallote
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so taking the derivative and equating to zero is another story?
Lukecrayonz
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It'll be exactly the same basically I think.
Lukecrayonz
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Yes lgb haha
tbrooks3
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they give us this freshman yr. imma senior now tryna make it up.. sucks :/
tbrooks3
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thanks everyone
anonymous
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using the derivative to find the minimum value of a quadratic. like using a glock to kill a wingspanroach