Найти критические точки f(x)=5+12x-7x³

[latex]f(x)=5+12x-7x^3[/latex] [latex]f'(x)=(5+12x-7x^3)'=12-21x^2[/latex] [latex]f'(x)=0[/latex] [latex]12-21x^2=0[/latex] [latex]21x^2=12[/latex] [latex]7x^2=4[/latex] [latex]x^2= \frac{4}{7} [/latex] [latex]x=б \sqrt{ \frac{4}{7} } [/latex] [latex]x=б \ \frac{2}{ \sqrt{7} } } [/latex] [latex]x=б \ \frac{2 \sqrt{7} }{7} } [/latex]