一、假設(R, +, • )是一環（ring）且a,b,c ∈ R。下列敘述何者為真？請寫出計算過程。（10 分） ⑴a + b ∈ R且a • b ∈ R ⑵a + (b + c) = (a + b) + c且a • (b • c) = (a • b) • c ⑶a + b = b + a且a • b = b • a ⑷a • (b + c) = (a • b) + (a • c)且(b + c) • a = (b • a) + (c • a) ⑸存在u ∈ R使得對於任意r ∈ R，r • u = u • r = r成立