Identify the quantifiers in the following statements.
(i) There exists a triangle which is not equilateral.
(ii) For all real numbers $x$ and $y, x y=y x$.
(iii) There exists a real number which is not a rational number.
(iv) For every natural number $x, x+1$ is also a natural number.
(v) For all real numbers $x$ with $x>3, x^2$ is greater than 9 .
(vi) There exists a triangle which is not an isosceles triangle.
(vii) For all negative integers $x, x^3$ is also a negative integers.
(viii) There exists a statement in above statements which is not true.
(ix) There exists an even prime number other than 2.
(x) There exists a real number $x$ such that $x^2+1=0$.
Quantifier are the phrases like 'There exist' and 'For every', 'For all' etc.
(i) There exists
(ii) For all
(iii) There exists
(iv) For every
(v) For all
(vi) There exists
(vii) For all
(viii) There exists
(ix) There exists
(x) There exists
Prove by direct method that for any integer ' $n$ ', $n^3-n$ is always even.
Here, two cases arise
Case I When $n$ is even,
$$\begin{aligned} & \text { Let } \quad n=2 K, K \in N \\ & \Rightarrow \quad n^3-n=(2 K)^3-(2 K)=2 K\left(4 K^2-1\right) \\ & =2 \lambda \text {, where } \lambda=K\left(4 K^2-1\right) \end{aligned}$$
Thus, $\left(n^3-n\right)$ is even when $n$ is even.
Case II When $n$ is odd,
$$\begin{aligned} \text{Let}\quad n & =2 K+1, K \in N \\ \Rightarrow \quad & n^3-n & =(2 K+1)^3-(2 K+1) \\ & =(2 K+1)\left[(2 K+1)^2-1\right] \\ & =(2 K+1)\left[4 K^2+1+4 K-1\right] \\ & =(2 K+1)\left(4 K^2+4 K\right) \\ & =4 K(2 K+1)(K+1) \\ & =2 \propto, \text { when } \alpha=2 K(K+1)(2 K+1) \end{aligned}$$
Then, $n^3-n$ is even when $n$ is odd.
So, $n^3-n$ is always even.
Check validity of the following statement.
(i) $p: 125$ is divisible by 5 and 7.
(ii) $q: 131$ is a multiple of 3 or 11 .
(i) $p: 125$ is divisible by 5 and 7 .
Let $q: 125$ is divisible by 5 .
$r: 125$ is divisible by 7 .
$q$ is true, $r$ is false.
$\Rightarrow q \wedge r$ is false.
[since, $p \wedge q$ has the truth value $F$ (false) whenever either $p$ or $q$ or both have the truth value F.]
Hence, $p$ is not valid.
(ii) $p: 131$ is a multiple of 3 or 11 .
Let $q: 131$ is multiple of 3 .
$r: 131$ is a multiple of 11 .
$p$ is true, $r$ is false.
$\Rightarrow \quad p \vee r$ is true. [since, $p \vee q$ has the truth value $T$ (true) whenever either $p$ or $q$ or both have the truth value T]
Hence, $q$ is valid.
Prove the following statement by contradiction method
$p$ : The sum of an irrational number and a rational number is irrational.
Let $p$ is false i.e., sum of an irrational and a rational number is rational.
Let $\sqrt{m}$ is irrational and $n$ is rational number.
$\Rightarrow\quad\sqrt{m}+n=r\quad \text{[rational]}$
$\Rightarrow\quad\sqrt{m}=r-n$
$\sqrt{\mathrm{m}}$ is irrational, where as $(r-n)$ is rational. This is contradiction.
Then, our supposition is wrong.
Hence, $p$ is true.
Prove by direct method that for any real number $x, y$ if $x=y$, then $x^2=y^2$.
Let $p: x=y, \quad x, y \in R$
On squaring both sides,
$$\begin{gathered} x^2=y^2: q \quad\text{[say]}\\ p \Rightarrow q \end{gathered}$$
Hence, we have the result.