Maths Olympiad Prep

Library / /158 of 520

Algebra Difficulty 3.0 Junior Find the answer

The negation of the proposition "There exists xRx \in \mathbb{R}, such that x2+2x+5=0x^2 + 2x + 5 = 0" is.

A number or a short expression. Spacing and $ signs are ignored.

Solutions — 2

Solution 1

For any xRx \in \mathbb{R}, it holds that x2+2x+50x^2 + 2x + 5 \neq 0.

Thus, the negation of the given proposition is For any xR,x2+2x+50\boxed{\text{For any } x \in \mathbb{R}, x^2 + 2x + 5 \neq 0}.

Solution 2

To negate the proposition "There exists xRx \in \mathbb{R} such that x2+2x+5=0x^{2}+2x+5=0", we recognize that it is an existential statement. The negation of an existential statement is a universal statement, which means that the claim is false for all elements in the domain.

Hence, the negation of the given proposition is: "For all xRx \in \mathbb{R}, it holds that x2+2x+50x^{2}+2x+5 \neq 0."

Let's strengthen the proof by reasoning why this must be true. The given equation is a quadratic equation. The discriminant of this quadratic equation, derived from coefficients a=1a=1, b=2b=2, and c=5c=5, is Δ=b24ac=(2)2415=420=16\Delta = b^2 - 4ac = (2)^2 - 4 \cdot 1 \cdot 5 = 4 - 20 = -16. Since the discriminant is negative, the equation has no real solutions. This justifies that the original statement is indeed false for all real numbers xx, thereby confirming the correctness of the negated statement.

In conclusion, the negation is: For all xR,x2+2x+50\boxed{\text{For all } x \in \mathbb{R}, x^{2}+2x+5 \neq 0}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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