Theo đề bài, ta có:
ab=cd
→ab-2=cd-2
→ab-2b=cd-2d
→a-2b=c-2d
Đặt ab=cd=k(k≠0)
=>a=bk,c=dka−2bb=bk−2bb=k−2c−2dd=dk−2dd=k−2=>a−2bb=c−2dd
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