by viet_jon
 P: 135 1. The problem statement, all variables and given/known data solve (f(x)=-3 x^2 + 9 x + 1/4 ) fx=(-3)(x^2-3 x)+1/4 fx=(-3)(x^2-3 x+9/4)+1/4+27/4 fx=(-3)(x^2-3 x+9/4)+28/4 fx=(-3) (x-3/2)^2+7 2. Relevant equations 3. The attempt at a solution the solution is already given in the book, but I don't understand why 9/4 was inserted in the third line (bolded). I know at the end of the equation, 27/4 is added to balance out the 9/4.......but why put in 9/4 there?
 Emeritus Sci Advisor PF Gold P: 4,975 What they've done is simply completed the square. If you have an equation of the form: $x^2+2ax$, you can write it as: $x^2+2ax = (x+a)^2 - a^2$.
 HW Helper P: 6,207 f(x)=-3 x^2 + 9 x + 1/4 they just put 27/4 and -27/4 to make completion easier f(x)=-3 x^2 + 9 x -27/4+ 1/4 +27/4 =-3(x^2-3x+9/4)+7 =-3(x-3x/2)^2 +7 expand (x-3x/2)^2 and check it yourself
P: 135

yea bro...

the thing is, how do you come up with 9/4 from to make it easier?

I'm on purplemath.com right now, trying to figure it out without bothering you guys... hopefully I can figure this one out.
 Emeritus Sci Advisor PF Gold P: 4,975 Well heres how I would have done that problem. You're given: $$f(x) = -3x^2 + 9x + \frac{1}{4}$$ $$f(x) = -3( x^2 -3x) + 1/4$$ Now we notice that the term in brackets is of the form $x^2+2ax$, with $a=\frac{-3}{2}$, and so we complete the square $x^2+2ax = (x+a)^2 - a^2$: $$f(x) = -3\left(\left(x-\frac{3}{2}\right)^2 - \frac{9}{4} \right) + \frac{1}{4}$$ $$f(x) = -3\left(x - \frac{3}{2}\right)^2 + \frac{27}{4} + \frac{1}{4}$$ $$f(x) = -3\left(x - \frac{3}{2}\right)^2 + 7$$
 P: 135 okie.........i got it. so the point of adding -9/4 inside the brackets is so that you can make it factorable. And since you added a (-), now you have to minus it to even it out. thnkx.....can't believe it took me so long. anyhow, how do you type the math out like that?
 P: 8 You use LaTeX. This tread explains in more detail. http://www.physicsforums.com/showthread.php?t=8997
 P: 135 test: $$x=-3 x^2+9x+1/4$$ $$fx=(-3)(x^2┤-3 x)+1/4\\$$ $$fx=(-3)(x^2┤-3 x+9/4)+1/4+27/4\\$$ $$fx=(-3)(x^2┤-3 x+9/4)+28/4\\$$ $$fx=(-3) (x┤-3/2)^2+7\\$$
Emeritus