as f(x) is a polynomial of degree two let it be f(x)=ax2+bx+c(a=0) on satisfying given conditions we get C=1&a=±1 hence f(x)=1±x2 also range ∈(−∞,1] hence f(x)=1−x2 now f(k)=−2k 1−k2=−2k→k2−2k−1=0 let roots of this equation be α&β then α2+β2=(α+β)2−2αβ =4−2(−1)=6