# need help fotr math

• Jun 10th 2008, 05:43 PM
tejashtaj
need help fotr math
Quote:

Originally Posted by Bajan
Hey guys, having trouble with this problem :-/

how to find a quadratic equation is standard from whoes rooot are given by

r₁=3+√2 and r₂=3-[FONT='Arial','sans-serif']√2
[/FONT]
• Jun 10th 2008, 06:15 PM
topsquark
Quote:

Originally Posted by tejashtaj
how to find a quadratic equation is standard from whoes rooot are given by

r₁=3+√2 and r₂=3-[FONT='Arial','sans-serif']√2[/font]

You know the values of the two roots, and we know that one form of the quadratic for this will be given by
$(x - r_1)(x - r_2)$

Thus
$(x - (3 + \sqrt{2} ))(x - (3 - \sqrt{2}))$

The simplest way to expand this (in my opinion) is to write this as
$((x - 3) - \sqrt{2} ) ((x - 3) + \sqrt{2})$
which is in the form of $(a - b)(a + b) = a^2 - b^2$,

$((x - 3) - \sqrt{2} ) ((x - 3) + \sqrt{2}) = (x - 3)^2 - (\sqrt{2})^2$

$= x^2 - 6x + 9 - 2 = x^2 - 6x + 7$
as you can check.

-Dan
• Jun 10th 2008, 06:40 PM
tejashtaj
write a quadratic of the form f(x)=a[FONT='Arial','sans-serif']x² +bx +c for the quadratic function whose graph passes through the point (o,8),(8,0) and (-2,5).[/FONT]

[FONT='Arial','sans-serif'](Hint find the values of a,b and c!)[/FONT]
• Jun 10th 2008, 08:04 PM
Reckoner
Quote:

Originally Posted by tejashtaj
write a quadratic of the form f(x)=a[FONT='Arial','sans-serif']x² +bx +c for the quadratic function whose graph passes through the point (o,8),(8,0) and (-2,5).[/font]

[FONT='Arial','sans-serif'](Hint find the values of a,b and c!)[/font]

Substitute the given values into the general equation, and you will end up with a system of linear equations in $a,\;b,\text{ and }c$:

$\begin{array}{rcl}
(x,\;y) &\rightarrow& y = ax^2 + bx + c\\
(0,\;8) &\rightarrow& 8 = a(0)^2 + b(0) + c\\
(8,\;0) &\rightarrow& 0 = a(8)^2 + b(8) + c\\
(-2,\;5) &\rightarrow& 5 = a(-2)^2 + b(-2) + c
\end{array}$

and you get the following system:

$\left\{\begin{array}{rcrcrcr}
64a & + & 8b & + & c & = & 0\\
4a & - & 2b & + & c & = & 5\\
&&&& c & = & 8
\end{array}\right.$

Now solve for the needed coefficients.