If we choose n as a square say m^2 then
n(n+1) = m^2(m^2+1) = m^4 + m^2
now n(n+1) is reperesented as sum of 2 squares that is (m^2)^2 + m^2
if m^2 can be represented as sum of 2 squares that is p^2 + q^2 this is possible as as (m,p,q) form a pythagorean triple then we have
n(n+1) = (p^2+q^2)(m^2+1) = (pm+ q)^2 + (p-qm)^2 this is another way
if we chose p=x^2-y^2, q = 2xy, m= x^2+y^2, n = (x^2+y^2)^2 then it satisfies the condition (p,q,m) form a pythagrean triple)
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