|c-(a+b)|²=|c|²+|a+b|²-2c·(a+b)=|c²+2-2√2|c|cos=1即：cos=(|c|²+1)/(2√2|c|)∈[-1,1](|c|²+1)/(2√2|c|)≤1,可得：√2-1≤|c|≤√2+1(|c|²+1)/(2√2|c|)≥-1自动满足,不用...