C

\(a^2=x^2y^2+(1+x^2)(1+y^2)+2xy\sqrt{(1+x^2)(1+y^2)} \\->2xy\sqrt{(1+x^2)(1+y^2)}=a^2-2x^2y^2-1-x^2-y^2 \\E^2=x^2(1+y^2)+y^2(1+x^2)+2xy\sqrt{(1+x^2)(1+y^2)} \\=x^2+y^2+2x^2y^2+a^2-2x^2y^2-1-x^2-y^2 \\=a^2-1\)

\(a)2x(x+1)-2x^2=0 <=> 2x^2+2x-2x^2=0 \\<=>2x=0<=>x=0 \\b)3(x-7)+4x(x-7)=0<=>(4x+3)(x-7)=0 \\<=>4x+3=0\ hoặc\ x+7=0 \\<=>x=\dfrac{-3}{4}\ hoặc\ x=-7 \\c)x^2-x=12<=>x^2-x-12=0 \\<=>(x+3)(x-4)=0 \\<=>x+3=0\ hoặc\ x-4=0 \\<=>x=-3\ hoặc\ x=4\)

???

\(a^2+a^2(a+1)^2+(a+1)^2 \\=a^4+2a^3+3a^2+2a+1 \\=(a^2+a+1)^2 \) 

Thay a = 2018 ta được A chính phương.

3 số

Lỗi

??

\(a)\dfrac{5}{77}-\dfrac{-4}{7}=\dfrac{5}{77}+\dfrac{44}{77}=\dfrac{49}{77}=\dfrac{7}{11}\\b)\dfrac{4}{33}-\dfrac{6}{-11}=\dfrac{4}{33}+\dfrac{18}{33}=\dfrac{22}{33}=\dfrac{2}{3} \\c)\dfrac{25}{7}-\dfrac{61}{21}=\dfrac{75}{21}-\dfrac{61}{21}=\dfrac{14}{21}=\dfrac{2}{3} \\d)\dfrac{5}{10}-\dfrac{1}{2}=\dfrac{1}{2}-\dfrac{1}{2}=0\)

\(e)\dfrac{2}{3}-\dfrac{-5}{6}=\dfrac{4}{6}+\dfrac{5}{6}=\dfrac{9}{6}=\dfrac{3}{2} \\f)\dfrac{5}{6}-\dfrac{-1}{3}=\dfrac{5}{6}+\dfrac{2}{6}=\dfrac{7}{6}\)