原式=S(V1²+V2²+2V1*V2)-4S*V1*V2
=S*V1²+S*V2²+2S*V1*V2-4S*V1*V2
=S*V1²+S*V2²-2S*V1*V2
=S(V1²+V2²-2V1*V2)
=S(V1-V2)²
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