Y=3x^2+2x+6的单调性 写出增函数及减函数?

Y=3x^2+2x+6的单调性 写出增函数及减函数?

y=3x^2+2x+6
=3(x^2+2/3x)+6
=3[x^2+2/3x+1/9-1/9]+6
=3[(x+1/3)^2-1/9]+6
=3(x+1/3)^2-1/3+6
=3(x+1/3)^2+17/3
f(x)在（负无穷,-1/3）上为减函数,在（-1/3,正无穷）上为增函数
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