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Algebra Difficulty 4.5 AIME Prove it

Prove the following inequalities using the method of subtraction:(1)(1) For xR\forall x\in R, 2x2+5x+3>x2+3x+12x^{2}+5x+3 \gt x^{2}+3x+1;(2)(2) For a>b>0a \gt b \gt 0, a>b\sqrt{a} \gt \sqrt{b}.

Solution

Proof:

Part (1): To show 2x2+5x+3>x2+3x+12x^{2}+5x+3 > x^{2}+3x+1 for xR\forall x\in \mathbb{R},

First, subtract the right-hand side from the left-hand side:

(2x2+5x+3)(x2+3x+1)=2x2+5x+3x23x1=x2+2x+2=(x+1)2+1. \begin{align*} (2x^{2}+5x+3)-(x^{2}+3x+1) & = 2x^2 + 5x + 3 - x^2 - 3x - 1 \\ & = x^{2} + 2x + 2 \\ & = (x + 1)^{2} + 1. \end{align*}

Since for xR\forall x\in \mathbb{R}, we have (x+1)20(x+1)^{2} \geq 0, thus (x+1)2+1>0(x + 1)^{2} + 1 > 0.

Therefore, it follows that 2x2+5x+3>x2+3x+12x^{2}+5x+3 > x^{2}+3x+1.

Part (2): To show a>b\sqrt{a} > \sqrt{b} for a>b>0a > b > 0,

Consider the difference ab\sqrt{a}-\sqrt{b}:

ab=(ab)(a+b)a+b=aba+b. \begin{align*} \sqrt{a}-\sqrt{b} & = \frac{(\sqrt{a}-\sqrt{b})(\sqrt{a}+\sqrt{b})}{\sqrt{a}+\sqrt{b}} \\ & = \frac{a - b}{\sqrt{a} + \sqrt{b}}. \end{align*}

Given that a>b>0a > b > 0, it implies ab>0a - b > 0. Also, since a>0a > 0 and b>0b > 0, a+b>0\sqrt{a} + \sqrt{b} > 0.

Therefore, we have aba+b>0\frac{a-b}{\sqrt{a}+\sqrt{b}} > 0, which implies a>b\sqrt{a} > \sqrt{b}.

Thus, the proofs for both parts are as follows:

For part (1): 2x2+5x+3>x2+3x+12x^{2}+5x+3 > x^{2}+3x+1 is true for all xRx \in \mathbb{R}.

For part (2): a>b\sqrt{a} > \sqrt{b} for a>b>0a > b > 0.

Final conclusions are:

- For part (1): 2x2+5x+3>x2+3x+12x^{2}+5x+3 > x^{2}+3x+1 True\boxed{\text{True}}
- For part (2): a>b\sqrt{a} > \sqrt{b} True\boxed{\text{True}}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.