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\(1,\left(ac+bd\right)^2+\left(ad-bc\right)^2\\ =a^2c^2+2abcd+b^2d^2+a^2d^2-2abcd+b^2c^2\\ =a^2c^2+b^2d^2+a^2d^2+b^2c^2\\ =\left(a^2c^2+a^2d^2\right)+\left(b^2d^2+b^2c^2\right)\\ =a^2\left(c^2+d^2\right)+b^2\left(c^2+d^2\right)\\ =\left(a^2+b^2\right)\left(c^2+d^2\right)\)
2, \(\left(a^2+b^2\right)\left(c^2+d^2\right)\ge\left(ac+bd\right)^2\)
\(\Leftrightarrow a^2c^2+b^2c^2+a^2d^2+b^2d^2\ge a^2c^2+2abcd+b^2d^2\)
\(\Leftrightarrow b^2c^2-2abcd+a^2d^2\ge0\)
\(\Leftrightarrow\left(bc-ad\right)^2\ge0\)
Dấu "=" xảy ra \(\Leftrightarrow bc=ad\Leftrightarrow\dfrac{a}{b}=\dfrac{c}{d}\)
\(1\)/
⇔ \(\left(ac\right)^2+2abcd+\left(bd\right)^2+\left(ad\right)^2-2abcd+\left(bc\right)^2=\left(a^2+b^2\right)\left(c^2+d^2\right)\)
⇔\(a^2\left(c^2+d^2\right)+b^2\left(c^2+d^2\right)=\left(a^2+b^2\right)\left(c^2+d^2\right)\)
⇔\(\left(a^2+b^2\right)\left(c^2+d^2\right)=\left(a^2+b^2\right)\left(c^2+d^2\right)\) ⇒ \(\left(dpcm\right)\)
\(2\)/
⇔\(\left(ac\right)^2+\left(ad\right)^2+\left(bc\right)^2+\left(bd\right)^2\ge\left(ac\right)^2+2abcd+\left(bd\right)^2\)
⇔\(\left(ad\right)^2-2abcd+\left(bc\right)^2\ge0\)
⇔\(\left(ad-bc\right)^2\ge0\left(đúng\right)\)
a: \(VT=a^2c^2+2abcd+b^2d^2+a^2d^2-2abcd+b^2c^2\)
\(=a^2c^2+a^2d^2+b^2d^2+b^2c^2\)
\(=a^2\left(c^2+d^2\right)+b^2\left(c^2+d^2\right)\)
\(=\left(c^2+d^2\right)\left(a^2+b^2\right)\)
b: Bạn ghi lại đề đi bạn
a: \(\left(ac+bd\right)^2+\left(ad-bc\right)^2\)
\(=a^2c^2+b^2d^2-2abcd+a^2d^2-2abcd+b^2c^2\)
\(=a^2c^2+a^2d^2+b^2d^2+b^2c^2\)
\(=\left(c^2+d^2\right)\left(a^2+b^2\right)\)
b: \(\left(ac+bd\right)^2< =\left(a^2+b^2\right)\left(c^2+d^2\right)\)
\(\Leftrightarrow a^2c^2+2abcd+b^2d^2-a^2c^2-a^2d^2-b^2c^2-b^2d^2< =0\)
\(\Leftrightarrow-a^2d^2+2abcd-b^2c^2< =0\)
\(\Leftrightarrow\left(ad-bc\right)^2>=0\)(luôn đúng)
a) \(\left(ac+bd\right)^2+\left(ad-bc\right)^2\)
\(=a^2c^2+2abcd+b^2d^2+a^2d^2-2adbc+b^2c^2\)
\(=a^2c^2+b^2d^2+a^2d^2+b^2c^2\)
\(=\left(a^2c^2+a^2d^2\right)+\left(b^2d^2+b^2c^2\right)\)
\(=a^2\left(c^2+d^2\right)+b^2\left(c^2+d^2\right)\)
\(=\left(a^2+b^2\right)\left(c^2+d^2\right)\)
b) \(\left(a^2+b^2\right)\left(c^2+d^2\right)-\left(ac+bd\right)^{^2}\)
\(=a^2c^2+a^2d^2+b^2c^2+b^2d^2-a^2c^2-2abcd-b^2d^2\)
\(=a^2d^2+b^2c^2-2abcd\)
\(=\left(ad\right)^2-2ad.bc+\left(bc\right)^2\)
\(=\left(ad-bc\right)^2\ge0\)
\(=\left(ac+bd\right)^2\le\left(a^2+b^2\right)\left(c^2+d^2\right)\)
ta có ĐPCM
\(\left(ac+bd\right)^2+\left(ad-bc\right)^2=\left(a^2+b^2\right)\left(c^2+d^2\right)\)
<=> \(a^2c^2+2abcd+b^2d^2+a^2d^2+b^2c^2-2abcd=a^2c^2+a^2d^2+b^2c^2+b^2d^2\)
<=> \(a^2b^2+a^2d^2+b^2c^2+b^2d^2=a^2c^2+a^2d^2+b^2c^2+d^2b^2\) (luôn đúng )
b) ta có BĐT cần chứng minh \(\left(ax+by\right)^2< =\left(a^2+b^2\right)\left(x^2+y^2\right)\)
<=> \(a^2x^2+2axby+b^2y^2< =a^2x^2+a^2y^2+b^2x^2+b^2y^2\)
<=> \(0< =a^2y^2-2axby+b^2x^2\)
<=> \(\left(ay-bx\right)^2>=0\) (luôn đúng )
Rút gọn các kiểu nha bạn, EZ nên nhác làm quá
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