xi=λai+μbi,i=1,2,对于α∈(0,1),因A,B为凸集,故αa1+(1-α)a2=a∈A,αb1+(1-α)b2=b∈B,从而有 αx1+(1-α)x2=αλa1+αμb1+(1-α)λa2+(1-α)μb2 =λ(αa1+(1-α)2)+μ(αb1+(1-α)b2) =λa+μb∈λA+μB故λA+μB是凸集
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