Câu 1:
1/ A=($\frac{1}{2}$ -1)($\frac{1}{3}$ -1)...($\frac{1}{2021}$ -1) = $\frac{-1}{2}$ $\frac{-2}{3}$ ...$\frac{-2020}{2021}$ = $\frac{1}{2021}$
2/ a = (-$\frac{1}{5}$) $^{84}$ = ($\frac{1}{5}$) $^{2.42}$ = ($\frac{1}{25}$)
$^{b = (-$\frac{1}{3}$ )$^{126}$ = ($\frac{1}{3}$) $^{3.42}$ = ($\frac{1}{27}$) $^{42}$ 42}$
Ta thấy $\frac{1}{25}$>$\frac{1}{27}$
=> a>b
Câu 2:
1) |$x^{2}$ -25|$\geq$ 0 và |y-1|$\geq$ 0
Dấu "=" xảy ra <=> $x^{2}$ -25 =$\left \{ {{x^2-25=0} \atop {y-1=0}} \right.$ <=>$\left \{ {{x=5hoặc-5} \atop {y=1}} \right.$
