Wednesday

July 30, 2014

July 30, 2014

Posted by **robin!** on Tuesday, February 26, 2013 at 11:36am.

- algebra -
**Navneet**, Thursday, February 28, 2013 at 12:43pmThere exists only one solution and the whole challenge is in proving the fact that there does not exist any other solution. So here we go...

Let's add both these expressions. Then as per the problem,

7(a^2+b^2)=c^2+d^2

Clearly LHS is a multiple of 7 and so RHS must be a multiple of 7 in order for the solution to exist. Now, let's investigate the RHS.

Both c and d can be expressed as 7k, 7k+1, 7k+2,7k+3,7k+4, 7k+5,7k+6.

So, remainder when (c^2+d^2) is divided by 7 can be the same if one among the following is divided by7:

0, (1+2^2), (1+3^2), (1+4^2), (1+5^2), (1+6^2)

(2^2+3^2), (2^2+4^2), (2^2+5^2), (2^2+6^2)

(3^2+4^2), (3^2+5^2), (3^2+6^2)

(4^2+5^2), (4^2+6^2)

(5^2+6^2)

None of the above except 0 is divisible by 7.

Hence there exists only one solution for (a,b) -> (0,0)

- algebra -
**Calvin Lin**, Monday, March 4, 2013 at 1:19pmThis is a problem posted on Brilliant(dot)org

The above solution is wrong / incomplete.

-Calvin Lin

Brilliant Challenge Master

**Related Questions**

algebra - How many pairs of integers (not necessarily positive) are there such ...

algebra - How many pairs of integers (not necessarily positive) are there such ...

Number Theory.,Math - How many ordered pairs of positive integers (a,b) are ...

number theory, math - How many ordered pairs of positive integers (a,b) are ...

geometry!!! please help me!!!! - Determine the least positive integer n for ...

math, algebra - 2a+2ab+2b I need a lot of help in this one. it says find two ...

MATH combinatorics HELP!!!!! - Determine the least positive integer n for which ...

math - there are three consecutive positive integers such that the sum of the ...

Math (algebra) - Let x and y be real numbers satisfying 4x^2+5y^2=1. Over all ...

algebra - Find the sum of all positive integers c such that for some positive ...