Y=3x/ (x^2+1)
y'= [3x'(x^2+1)-3x(x^2+1)'] / (x^2+1)^2= [3(x^2+1)-6x^2]/(x^2+1)^2=
=(3x^2+3-6x^2)/(x^2+1)^2= (3-3x^2)/ (x^2+1)^2= [3(1-x^2)] /(x^2+1)^2;
y=0 => 3(1-x^2) / (x^2+1)^2=0
3(1-x^2)=0
(1-x)(1+x)=0
x=-1; x=1
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min. max.
y(-1)= -1,5
y(1)=1,5