1題f.4函數 望解答

f(k) = k^2 + (k+1)k + k ;
f(-k) = (-k)^2 + (k+1)(-k) + k = k^2 - k^2 - k + k = 0 ;
f(k) + f(-k) = 4
f(k) + 0 = 4
k^2 + (k+1)k + k = 4
k^2 + k^2 + k + k = 4
k^2 + k - 2 = 0
(k - 1)(k + 2) = 0
k = 1 or k = - 2