## anonymous one year ago f(x)=-1/2x^2+7x-2. x=7. Im trying to find the vertex, one is 7, and the other is 45/2. I keep getting 49/2. what am I missing?

1. anonymous

what does "one is 7" mean?

2. anonymous

first coordinate of the vertex is always $$-\frac{b}{2a}$$ in your case $$a=-\frac{1}{2},b=7$$

3. anonymous

ooh i see "one is 7" means the first coordinate is 7

4. anonymous

$f(7)=-\frac{1}{2}\times 7^2+7\times 7-2$

5. anonymous

that was supposed to be x=7 not one is 7

6. anonymous

i see exactly what you are missing you did not subtract 2 at the end

7. anonymous

if I subtract 2 from $\frac{ 49 }{ 2}$ how does that equal $\frac{ 45 }{ 2 }$?

8. anonymous

?

9. anonymous

the vertex is (7, 45/2). I cant figure out how its not 49/2. the class im in offers no explanation as to how it gets the answers, so im floored on how 7x7 equals 45.

10. anonymous

ok lets go slow

11. anonymous

did you get that you find the second coordinate of the vertex by finding $f(7)=-\frac{1}{2}\times 7^2+7\times 7-2$

12. anonymous

yes, I got that far find and this is what I did, in order (7)^2=49. 7x7=49-2=47 so then I have $-\frac{ 1 }{ 2 }(49)+47$

13. anonymous

you lost me entirely

14. anonymous

$-\frac{1}{2}\times 7^2+7\times 7-2\\ -\frac{1}{2}\times 49+47-2\\ -\frac{49}{2}+\frac{98}{2}-2\\ \frac{49}{2}-2\\ \frac{45}{2}$

15. anonymous

typo there , the second line should have 2 49's in it

16. campbell_st

just remember $-2 =- \frac{4}{2}$

17. campbell_st

so you have $\frac{49}{2} - \frac{4}{2} = \frac{45}{2}$

18. anonymous

okay thank you I didn't know how to get the second fraction. I see it know. I wish my class program would just explain it seesh. thanks again!