cheap answer, recall that minus * minus = plus. So if we just use any even exponent for (-1), that'll give us a positive.
(-1)² = (-1)(-1) = +1
(-1)⁴ = (-1)(-1)(-1)(-1) = 1
(-1)¹⁰ = (-1)(-1)(-1)(-1)(-1)(-1)(-1)(-1)(-1)(-1) = 1.
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