Everything Prime

Algebra Level 3

A quadratic equation with integral coefficients has two different prime numbers as its roots. If the sum of the coefficients of the equation is prime, then the sum of the roots is


The answer is 5.

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2 solutions

Rishabh Tripathi
Mar 5, 2016

Let the roots of required quadratic equation be α , β \alpha , \beta ( Where α , β \alpha , \beta are prime numbers.) So, the equation can be written as

( x α ) ( x β ) (x-\alpha)(x-\beta) = x 2 ( α + β ) + α β =x^{2}-(\alpha+\beta)+\alpha\beta

Since the sum of coefficients of the equation is prime:

1 α β + α β 1-\alpha-\beta+\alpha\beta is prime

( α 1 ) ( β 1 ) (\alpha-1)(\beta-1) is prime

The product of two numbers can be prime if and only if one of the numbers is 1 1

So, α 1 = 1 \alpha-1=1

α = 2 \alpha=2

Now,

( β 1 ) (\beta-1) is prime

also β \beta is prime

So, we're talking about two consecutive prime numbers and fortunately we have just one pair of consecutive prime numbers, i.e., ( 2 , 3 ) (2 , 3)

β = 3 \beta=3

Therefore, sum of roots of the equation = α + β = 2 + 3 = 5 =\alpha+\beta = 2+3 = 5

Thank you...amazing..

SURAJ KARAN - 2 years, 9 months ago

Let the quadratic polynomial be denoted by f (x) . Now sum of coefficients =f (1)=a prime m. Let primes p,q be such that f (p)=f (q)=0. Thus p-1 divides m and so does q-1. Since m is a prime, one of p & q must be equal to 2. We note that m must be 2. So, the other root must be 3. So, sum of roots is 5.

The only quadratic equation with prime roots p, q is (x - p) (x -q) = 0 whose coefficients are 1, - (p + q), pq. Sum of the coefficients = (p - 1)(q -1) = r say. r being a prime, it's only factors are 1 and r. So either p - 1= 1 or q - 1 = 1 (not both). Say p - 1= 1. Thus p = 2 and q - r = 1. The only primes satisfying the second equation are q =3 and r = 2. Hence p + q = 5.

Vijaya Prasad Nalluri - 3 years, 10 months ago

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