Question #179832

Prove that 𝑓(𝑥) = 𝑥 2 + 2𝑥 is not injective


1
Expert's answer
2021-04-29T16:47:12-0400

Given,

f(x)=x2+2xf(x)=x^2+2x


x2+2xy=0x^2+2x-y=0


The solution of the above equation is-


x=2±4+4y2x=\dfrac{-2\pm\sqrt{4+4y}}{2}


x=1±1+yx=-1\pm\sqrt{1+y}


which is defined in R for y1.y≥−1. This can be used to separate the domain of f(x) into two intervals. For x[1,x\in[1,∞ ) and x(,1]x\in(−∞,1] , the function f(x) is not injective (and invertible). The domain that contains 2 is of course [1,).[1,∞).


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