The steps of finding a function period F (1-x) + F (1 + x) = 0, Y = f (x) is an even function What's the sixth step? I don't understand

Let 1-x = t
f(t)+f(2-t)=0
f(t)+f(t-2)=0
Let T-2 = y
f(y+2)+f(y)=0
Let y = y + 2
f(y+4)+f(y+2)=0
f(y)=f(y+4)
Period n = 4

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