A=,x²-4xy-4y² +3

= (x-2y)²+3

do ( x-2y)²\(\ge0\forall x;y\)

=> (x-2y)²+3\(\ge3\)

=> A\(\ge3\)

vậy A \(>0\) với mọi số thực x;y

4-56=-52

(-6) + 5 = -1

e) 2^x+1=64

=>2^x+1=2⁶

=> x+1=6

=>x=5

d) 2x³=54

=> x³=54:2

=> x³=27

=> x³=3³

=>x=3

a)4x⁴+36x²+81-36x²

=(4x⁴+36x²+81)-63x²

=(2x²+9)²-36x²

=(2x²+9-6x)(2x²+9+6x)

;
để \(27x^2+a⋮3x+2\) thì

a+12=0

a=-12

n(n+1)=6

=> n²+n-6=0

=> n²+3n-2n-6=0

=> (n²+3n)-(2n+6)=0

=>n(n+3)-2(n+3)=0

=> (n+3)(n-2)=0

=>\(\left[{}\begin{matrix}n+3=0\\n-2=0\end{matrix}\right.\)=>\(\left[{}\begin{matrix}n=-3\\n=2\end{matrix}\right.\)

vậy n\(\in\) {-3;2}