18 inviting

19 lovely

20 friendly

21 helpful

22 funny

j vậy

11 C

12 A

13 B

14 C

15 A

16 B

17 A

18 C

19 C

20 C

Heo chuồng

a,=x4+2x2y+y2b,=[(y−x)3+(x−y)3]+[(x+y)3−x3−y3]=[−(x−y)3+(x−y)3]+(x3+3x2y+3xy2+y3−x3−y3)=0+3x2y+3xy2=3x2y+3xy2c,=x3+1−x2−x3=1−x2d,=[(x+y)2+z2]=(x+y)2+2z(x+y)+z2=x2+y2+z2+2xy+2yz+2zx

A=√50−3√12=2√5−3√2=2√5−3√22=4√5−3√22

⇒x2=3⋅12=36⇒[x=6x=−6