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Oct 13, 2016
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Write a rule for a quadratic function with a graph that has x-intercepts (2,0) and (-6,0) and a maximum point of (-2,4)

 

quadratic function: \(y =ax^2+bx+c\)

 

1. Point (x=2, y=0):

\( \quad 0 = a\cdot (2^2)+b\cdot 2 + c \\ \Rightarrow 4a+2b+c=0\)

 

Maximum Point (\(x_{max} = -2, \ y_{max} = 4\)):

\(\begin{array}{|rcll|} \hline x_{max} &=& -\frac{b}{2a} \\ -2 &=& -\frac{b}{2a} \\ 2 &=& \frac{b}{2a} \\ 4a &=& b \\ \mathbf{ a } & \mathbf{=}& \mathbf{\frac{b}{4} }\\ \hline \end{array}\)

 

\(\begin{array}{|rcll|} \hline 4a+2b+c&=&0 \quad | \quad a=\frac{b}{4} \\ 4\cdot \frac{b}{4} +2b+c&=&0 \\ b+2b+c&=&0 \\ 3b+c&=&0 \\ 3b&=&-c \\ \mathbf{ b } & \mathbf{=}& \mathbf{-\frac{c}{3}} \\ \hline \end{array}\)

 

\(\begin{array}{|rcll|} \hline a &=& \frac{b}{4} \quad | \quad b = -\frac{c}{3} \\ a &=& \frac{ -\frac{c}{3}}{4} \\ a &=& -\frac{c}{12} \\\\ y_{max} &=& a\cdot ( x_{max} ) ^2 + b\cdot x_{max} + c \quad | \quad a = -\frac{c}{12} \quad b = -\frac{c}{3} \\ y_{max} &=& -\frac{c}{12}\cdot ( x_{max} ) ^2 -\frac{c}{3}\cdot x_{max} + c \quad | \quad x_{max} = -2\\ y_{max} &=& -\frac{c}{12}\cdot ( -2 ) ^2 -\frac{c}{3}\cdot (-2) + c \quad | \quad y_{max} = 4\\ 4 &=& -\frac{1}{3}\cdot c +\frac{2}{3}\cdot c + c \\ 4 &=& \frac{4}{3}\cdot c \\ \mathbf{ c } & \mathbf{=}& \mathbf{3} \\\\ b&=&-\frac{c}{3}\\ \mathbf{ b } & \mathbf{=}& \mathbf{-1} \\\\ a&=&-\frac{b}{4}\\ \mathbf{ a } & \mathbf{=}& \mathbf{-\frac{1}{4}} \\ \hline \end{array}\)

 

quadratic function:

\(\begin{array}{|rcll|} \hline y &=& ax^2+bx+c \\ \mathbf{ y }&\mathbf{=}& \mathbf{-\frac{1}{4}x^2-x+3} \\ \hline \end{array}\)

 

 

laugh

Oct 13, 2016
 #1
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Expand the following:
(a-b+c-d) (a+b-c+d)

(a-b+c-d) (a+b-c+d) = a (a-b+c-d)+b (a-b+c-d)-c (a-b+c-d)+(a-b+c-d) d:
a (a-b+c-d)+b (a-b+c-d)-c (a-b+c-d)+d (a-b+c-d)

a (a-b+c-d) = a a+a (-b)+a c+a (-d):
a a-a b+a c-a d+b (a-b+c-d)-c (a-b+c-d)+d (a-b+c-d)

a a = a^2:
a^2-a b+a c-a d+b (a-b+c-d)-c (a-b+c-d)+d (a-b+c-d)

b (a-b+c-d) = b a+b (-b)+b c+b (-d):
a^2-a b+a c-a d+b a-b b+b c-b d-c (a-b+c-d)+d (a-b+c-d)

b (-b) = -b^2:
a^2-a b+a c-a d+b a+-b^2+b c-b d-c (a-b+c-d)+d (a-b+c-d)

-c (a-b+c-d) = -(c a)-c (-b)-c c-c (-d):
a^2-a b+a c-a d+b a-b^2+b c-b d+-(c a)-(-c b)-c c-(-c d)+d (a-b+c-d)

-(-1) = 1:
a^2-a b+a c-a d+b a-b^2+b c-b d-c a+c b-c c-(-c d)+d (a-b+c-d)

-c c = -c^2:
a^2-a b+a c-a d+b a-b^2+b c-b d-c a+c b+-c^2-(-c d)+d (a-b+c-d)

-(-1) = 1:
a^2-a b+a c-a d+b a-b^2+b c-b d-c a+c b-c^2+c d+d (a-b+c-d)

d (a-b+c-d) = d a+d (-b)+d c+d (-d):
a^2-a b+a c-a d+b a-b^2+b c-b d-c a+c b-c^2+c d+d a-d b+d c-d d

d (-d) = -d^2:
a^2-a b+a c-a d+b a-b^2+b c-b d-c a+c b-c^2+c d+d a-d b+d c+-d^2

Grouping like terms, a^2-a b+a c-a d+b a-b^2+b c-b d-c a+c b-c^2+c d+d a-d b+d c-d^2 = -d^2+(c d+c d)+(-(b d)-b d)-c^2+(b c+b c)-b^2+a^2+(a d-a d)+(a c-a c)+(a b-a b):
-d^2+(c d+c d)+(-(b d)-b d)-c^2+(b c+b c)-b^2+a^2+(a d-a d)+(a c-a c)+(a b-a b)

c d+c d = 2 (c d):
-d^2+2 c d+(-(b d)-b d)-c^2+(b c+b c)-b^2+a^2+(a d-a d)+(a c-a c)+(a b-a b)

-(b d)-b d = -2 (b d):
-d^2+2 c d+-2 b d-c^2+(b c+b c)-b^2+a^2+(a d-a d)+(a c-a c)+(a b-a b)

b c+b c = 2 (b c):
-d^2+2 c d-2 b d-c^2+2 b c-b^2+a^2+(a d-a d)+(a c-a c)+(a b-a b)

a d-a d = 0:
-d^2+2 c d-2 b d-c^2+2 b c-b^2+a^2+(a c-a c)+(a b-a b)

a c-a c = 0:
-d^2+2 c d-2 b d-c^2+2 b c-b^2+a^2+(a b-a b)

a b-a b = 0:
Answer: |-d^2 + 2cd - 2bd - c^2 + 2bc - b^2 + a^2

Oct 13, 2016

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