Q. Suppose a, b and c are integers. If a2+b2=c2 (i.e., a2+b2 is a perfect square), show that a or b is even.
Assume odd integers: Assume both a and b are odd integers.
Express a and b: Let a=2k+1 and b=2m+1, where k and m are integers.
Calculate a2: Calculate a2: (2k+1)2=4k2+4k+1.
Calculate b2: Calculate b2: (2m+1)2=4m2+4m+1.
Add a2 and b2: Add a2 and b2: 4k2+4k+1+4m2+4m+1.
Simplify the sum: Simplify the sum: 4k2+4k+4m2+4m+2.
Factor out common factor: Factor out the common factor of 4: 4(k2+k+m2+m)+2.
Identify contradiction: Notice that the sum is 2 more than a multiple of 4, which cannot be a perfect square since all perfect squares are either 0, 1, or 4 modulo 4.
Conclusion: Since our assumption leads to a contradiction, at least one of a or b must be even.