a + (b + c) = a b c + + = a b + c + = (a + b) + c
a - (b + c) = a b c + - = a b - c - = (a - b) - c
a + (b - c) = a b c - + = a b + c - = (a + b) - c
a - (b - c) = a b c - - = a b - c + = (a - b) + c
同じことが乗除についても成立する…と
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