(2а-1/2а)^2=4a^2-2*(2а*1/2а)+1/4a^2=4a^2+1/4a^2-2=9
4a^2+1/4a^2=11
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4/5=1/2+1/5+1/10
3/10=1/10+1/5
3/8=1/4+1/8
3/7=1/4+1/7+1/28
6/11=1/2+1/22