Let a, b, c and d be positive real numbers with a+b+c+d=2. Prove that ad+bc(a+c)2+ac+bd(b+d)2+4≥4(c+d+1a+b+1+a+b+1c+d+1).
Solution
According to Cauchy-Schwarz inequality (ad+bc(a+c)2+ac+bd(b+d)2+4)((ad+bc)+(ac+bd)+1)≥(a+c+b+d+2)2=16⟹ad+bc(a+c)2+ac+bd(b+d)2+4≥ad+bc+ac+bd+116. Set a+b=x, c+d=y (therefore x+y=2, x2+y2=4−2xy). It suffices to show that ad+bc+ac+bd+116=xy+116≥4(y+1x+1+x+1y+1). This is equivalent to 4(x+1)(y+1)⇔4(xy+3)⇔4xy+12⇔(xy)2+1⇔(xy−1)2≥(xy+1)((x+1)2+(y+1)2)≥(xy+1)(10−2xy)≥8xy−2(xy)2+10≥2xy≥0. Which is true, hence the claim of the problem. ■
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