\(a^2+b^2+c^2+d^2=a\left(b+c+d\right)\)
\(\Leftrightarrow4a^2+4b^2+4c^2+4d^2=4a\left(b+c+d\right)\)
\(\Leftrightarrow a^2-4ab+4b^2+a^2-4ac+4c^2+a^2-4ad+4d^2=0\)
\(\Leftrightarrow\left(a-2b\right)^2+\left(a-2c\right)^2+\left(a-2d\right)^2=0\)
\(\Leftrightarrow a=2b=2c=2d\)
=>A=\(a+\frac{a}{2}+\frac{a}{2}+\frac{a}{2}=\frac{5a}{2}\)
hàng 3 sử giúp mik hic hic
\(\Leftrightarrow a^2-4ab+4b^2+a^2-4ac+4c^2+a^2-4ad+4d^2+a^2=0\)
<=>(a-2b)2+(a-2c)2+(a-2d)2+a2=0
<=>a=2b=2c=2d=0
=>a=b=c=d=0
=>A=0+0+0+0=0
Hình như dòng 3 bn sai phải là
\(2a^2-4ab+4b^2+a^2-4ac+4c^2+a^2-4ad+4d^2=0\)
\(a^2+b^2+c^2+d^2=a\left(a+c+d\right)\)
\(\Rightarrow4a^2+4b^2+4c^2+4d^2=4a\left(b+c+d\right)\)
\(\Rightarrow a^2-4ab+4b^2+a^2-4ac+4c^2+a^2-4ad+4d^2\)=0
