tbrooks3 Group Title Find the minimum value for the function: y = x^2 + 9x + 18 2 years ago 2 years ago

1. samjordon Group Title

do u knwo how to complete the square? or do u factor it?

2. samjordon Group Title

lets factor its easy

3. tbrooks3 Group Title

its under my quadratic function lesson so idk

4. samjordon Group Title

what 2 numbers when you multiply them =18 and when u add them =9?

5. tbrooks3 Group Title

is there a number for that?

6. samjordon Group Title

6*3=18 6+3=9

7. tbrooks3 Group Title

ohhh

8. tbrooks3 Group Title

so is the answer 9 ?

9. samjordon Group Title

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

10. satellite73 Group Title

finding the zeros is a long and complicated way to find the minimum

11. samjordon Group Title

umm well we can complete the square

12. samjordon Group Title

but i dont think she knows that

13. Zarkon Group Title

use the vertex formula

14. satellite73 Group Title

minimum is at the vertex, and first coordinate of the vertex of $y=ax^2+bx+c$ is always $$-\frac{b}{2a}$$

15. tbrooks3 Group Title

when doing the vertx thing... is the second number the minimum?

16. satellite73 Group Title

in your case it is $$-\frac{9}{2}$$ replace $$x$$ by $$-\frac{9}{2}$$ to find the minimum

17. satellite73 Group Title

minimum means minimum value of $$y$$ so it is the second coordinate, not the first

18. satellite73 Group Title

so yes, what you said, the second number is the minimum.

19. samjordon Group Title

ohhh i like this method. We never learnt this stuff. Like the textbook always made us complete the square. oh cooll

20. tbrooks3 Group Title

can i just plug it into my graphing calculator to get the anwser ?

21. satellite73 Group Title

completing the square is for suckers, unless of course you are solving a quadratic equation

22. lgbasallote Group Title

last i heard minimum value was in a calculus class o.O i never knew it was in algebra too

23. Lukecrayonz Group Title

It's in Algebra 2.

24. Lukecrayonz Group Title

You can use a calculator to solve it

25. samjordon Group Title

hahah its in math 11 here in canada

26. satellite73 Group Title

$(-\frac{9}{2})^2+9\times (\frac{-9}{2})+18$ then a calculator if you need one

27. satellite73 Group Title

you turn math up to eleven in canada? mine only goes up to ten

28. Lukecrayonz Group Title

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.

29. Lukecrayonz Group Title

So for this, you get x=-4.5 and y=-2.25

30. tbrooks3 Group Title

sooo many numbers ... ahhhhh!!! >_< what about TI-83 plus?

31. samjordon Group Title

32. lgbasallote Group Title

so taking the derivative and equating to zero is another story?

33. Lukecrayonz Group Title

It'll be exactly the same basically I think.

34. Lukecrayonz Group Title

Yes lgb haha

35. tbrooks3 Group Title

they give us this freshman yr. imma senior now tryna make it up.. sucks :/

36. tbrooks3 Group Title

thanks everyone

37. satellite73 Group Title

using the derivative to find the minimum value of a quadratic. like using a glock to kill a wingspanroach