Quadratic Equation : Sum of Roots

Here is an interesting quadratic equations question. Combines some concepts of number theory.

Question
y = x^2 + bx + 256 cuts the x axis at (h, 0) and (k, 0). If h and k are integers, what is the least value of b?
A. –32
B. –256
C. –255
D. –257
E. 0

Correct Answer : Choice D. -257

Explanation

The curve cuts the x axis at (h, 0) and (k, 0). So, h and k are the roots of the quadratic equation.

For a quadratic equation of the form ax^2 + bx + c = 0, the sum of the roots = -b/a

The sum of the roots of this equation is  -b/1 = -b.Higher the sum of the roots lower the value of b.

For a quadratic equation of the form ax^2 + bx + c = 0, the product of roots = c/a.

Therefore, the product of the roots of this equation = 256/1 = 256.
i.e., h*k = 256

h and k are both integers.
So, h and k are both integral factors of  256.

256 can be expressed as product of two numbers in the following ways:
1 * 256
2 * 128
4 * 64
8 * 32
16 * 16


The sum of the roots is maximum when the roots are 1 and 256 and the maximum sum is 1 + 256 = 257.
The sum of the roots is minimum when the roots are -1 and -256 and the least sum is -1 - 256 = -257.

The least value possible for b is therefore -257.

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