Нужна помощь. помогите решить

#19:
$f(x) = x^4 - 4x^2 + 3 \\ f'(x) = 4x^3 - 8x = 4x(x^2 - 2)\\ f'(x) = 0 \ \textless \ =\ \textgreater \ x = 0$ ∪ $x = \sqrt2$ ∪ $x = -\sqrt2$
f(x)↓ (-∞; -√2] ∪ [0; √2]
f(x)↑ [-√2; 0] ∪ [√2; +∞)
max f(x) = 3
min f(x) = -1
#21:
$\left \{ {{x - y = 8} \atop {2^{x - 3y} = 16}} \right. \ \textless \ =\ \textgreater \ \left \{ {{x = 8 + y} \atop {2 ^ {8 - 2y} = 2^4}} \right. \ \textless \ =\ \textgreater \ \left \{ {{x = 8 + y} \atop {2y = 4}} \right. \ \textless \ =\ \textgreater \ \left \{ {{y=2} \atop {x=10}} \right.$
#22:
a) $4sin^2(x) = cos(x - \frac{ \pi }{2}) \ \textless \ =\ \textgreater \ 4sin^2(x) = sin(x) \ \textless \ =\ \textgreater \ \\ sin(x)(4sin(x)- 1) = 0 \ \textless \ =\ \textgreater \\$ $sin(x) = 0$ ∪ $sin(x) = \frac{1}{4}$
$\ \textless \ =\ \textgreater \ x = \pi k, k$ ∈ Z ∪ $x = +- arcsin(\frac{1}{4}) + 2\pi k, k$ ∈ Z
b) {2π; 2π + arcsin(1/4)}