Not so easy

Algebra Level 3

The quadratic polynomial p ( x ) p(x) has the following properties p ( x ) 0 p(x) \geq 0 for all real numbers, p ( 1 ) = 0 , p ( 2 ) = 2 p(1)=0 ,p(2)=2 . find the value of p ( 0 ) + p ( 3 ) p(0) + p(3) .


The answer is 10.

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2 solutions

Chew-Seong Cheong
Jan 17, 2015

It is given that the quadratic polynomial p ( x ) 0 p(x) \ge 0 for all real x x and that p ( 1 ) = 0 p(1) = 0 . This means that the polynomial is of the form: p ( x ) = a ( x 1 ) 2 \space p(x) = a(x-1)^2 .

Since p ( 2 ) = a ( 2 1 ) 2 = 2 a = 2 \space p(2) = a(2-1)^2 = 2 \quad \Rightarrow a = 2 .

Therefore, p ( 0 ) = 2 ( 0 1 ) 2 = 2 \space p(0) = 2(0-1)^2 = 2\space and p ( 3 ) = 2 ( 3 1 ) 2 = 8 \space p(3) = 2(3-1)^2 = 8 ,

p ( 0 ) + p ( 3 ) = 2 + 8 = 10 \quad \Rightarrow p(0) + p(3) = 2 + 8 = \boxed{10} .

Rishu Agarwal
Feb 8, 2015

let f ( x ) = a x ^(2)+b x +c then

f (1)=a+b+c=0 -------- (1)

f (2)=4a+2b+c=2 ------- (2)

Since p (x)>=0 , hence D >=0

=> b^(2) - 4ac>=0

=> (a+c)^(2) - 4ac >=0 ( Using eq 1 )

=> (a-c)^(2) >=0

which implies either a=c or a>c

taking a=c and solving equation 1 and 2

we get,

a=c=2 ; and b= -4

substitute the values in p (x) we get desired ans which is 10 :)

for p(x) to lie entirely above x axis and also for it to be equal to 0...it must have only one root.....hence D is always equal to 0.

Aman Kumar - 5 years, 11 months ago

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