ab(a+b)+bc(b+c)+ca(c+a)+3abc
= {ab(a+b)+ab•c｝+｛bc(b+c)+bc•a｝+｛ca(c+a)+ca•b｝
=ab(a+b+c) + bc(a+b+c) + ca(a+b+c)
=(ab+bc+ca)(a+b+c)
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