Quadratic Primes

x 2 p x + q = 0 \large x^2 -px + q = 0

Consider the quadratic equation above where p p and q q are prime numbers.

Given that this equation has all positive integral roots, find the value of ( p 3 + q 3 ) ( p q + 1 ) (p^3 + q^3)(p - q +1) .


The answer is 70.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Ankush Tiwari
Aug 15, 2014

Let a a and b b be the roots of the equation.

a + b = p \Rightarrow a + b = p and a b = q ab =q

Since q q is prime, either of a a and b b must be 1 1 and the other equal to q q

Assume that a = 1 a = 1 and b = q b = q

q + 1 = p \Rightarrow q + 1 = p

From this equation its clear that either of p p or q q must be even and the other odd , but p p and q q are primes , so

p = 3 \Rightarrow p =3 and q = 2 q=2

( p 3 + q 3 ) ( p q + 1 ) = 70 \Rightarrow (p^3 + q^3)(p-q+1) = 70

Nice solution

Ujjwal Mani Tripathi - 6 years, 9 months ago

Good logical solution.Hope we will get such tricky problems in future!

Prabir Chaudhuri - 6 years, 9 months ago

Nice solution

Sazzad Hossain Rafi - 4 years, 11 months ago

Same here!

Rick B - 6 years, 5 months ago
Rwit Panda
Jun 27, 2015

It is given that the roots are positive and integral.

(x^{2} + q = px

Here, x has to be positive and integral. We observe that the square of x + (a prime number) is divisible by x to satisfy the equation.

That is only possible when x is 1 or equal to q for positive integral value.

This gives us that q+1=p or q^2 + q=p

When x=1, p=3 and q=2.

So answer for this question is 70.

hi panda!!!!! @RWIT PANDA

sakshi rathore - 5 years, 10 months ago

Log in to reply

Sorry I can't recognize u

Rwit Panda - 5 years, 10 months ago

Log in to reply

actually in school you studied previously i was also there but in different section..i know u

sakshi rathore - 5 years, 10 months ago

Log in to reply

@Sakshi Rathore R u talking of RIS navi mumbai?

Rwit Panda - 5 years, 10 months ago

Log in to reply

@Rwit Panda yes.....I'm right is it you???

sakshi rathore - 5 years, 10 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...