# NCERT Solutions for Class 11 Maths Maths Chapter 14

## Mathematical Reasoning Class 11

### Exercise 14.1 : Solutions of Questions on Page Number : 324

Q1 :

Which of the following sentences are statements? Give reasons for your answer.

(i) There are 35 days in a month.

(ii) Mathematics is difficult.

(iii) The sum of 5 and 7 is greater than 10.

(iv) The square of a number is an even number.

(v) The sides of a quadrilateral have equal length.

(vii) The product of (-1) and 8 is 8.

(viii) The sum of all interior angles of a triangle is 180Â°.

(ix) Today is a windy day.

(x) All real numbers are complex numbers.

(i) This sentence is incorrect because the maximum number of days in a month is 31. Hence, it is a statement.

(ii) This sentence is subjective in the sense that for some people, mathematics can be easy and for some others, it can be difficult. Hence, it is not a statement.

(iii) The sum of 5 and 7 is 12, which is greater than 10. Therefore, this sentence is always correct. Hence, it is a statement.

(iv) This sentence is sometimes correct and sometimes incorrect. For example, the square of 2 is an even number. However, the square of 3 is an odd number. Hence, it is not a statement.

(v) This sentence is sometimes correct and sometimes incorrect. For example, squares and rhombus have sides of equal lengths. However, trapezium and rectangles have sides of unequal lengths. Hence, it is not a statement.

(vi) It is an order. Therefore, it is not a statement.

(vii) The product of (-1) and 8 is (-8). Therefore, the given sentence is incorrect. Hence, it is a statement.

(viii) This sentence is correct and hence, it is a statement.

(ix) The day that is being referred to is not evident from the sentence. Hence, it is not a statement.

(x) All real numbers can be written as a Ã— 1 + 0 Ã— i. Therefore, the given sentence is always correct. Hence, it is a statement.

Q2 :

Give three examples of sentences which are not statements. Give reasons for the answers.

The three examples of sentences, which are not statements, are as follows.

(i) He is a doctor.

It is not evident from the sentence as to whom ‘he’ is referred to. Therefore, it is not a statement.

(ii) Geometry is difficult.

This is not a statement because for some people, geometry can be easy and for some others, it can be difficult.

(iii) Where is she going?

This is a question, which also contains ‘she’, and it is not evident as to who ‘she’ is. Hence, it is not a statement.

### Exercise 14.2 : Solutions of Questions on Page Number : 329

Q1 :

Write the negation of the following statements:

(i) Chennai is the capital of Tamil Nadu.

(ii) is not a complex number.

(iii) All triangles are not equilateral triangle.

(iv) The number 2 is greater than 7.

(v) Every natural number is an integer.

Q2 :

Are the following pairs of statements negations of each other?

(i) The number x is not a rational number.

The number x is not an irrational number.

(ii) The numberx is a rational number.

The number x is an irrational number.

Q3 :

Find the component statements of the following compound statements and check whether they are true or false.

(i) Number 3 is prime or it is odd.

(ii) All integers are positive or negative.

(iii) 100 is divisible by 3, 11 and 5.

### Exercise 14.3 : Solutions of Questions on Page Number : 334

Q1 :

For each of the following compound statements first identify the connecting words and then break it into component statements.

(i) All rational numbers are real and all real numbers are not complex.

(ii) Square of an integer is positive or negative.

(iii) The sand heats up quickly in the Sun and does not cool down fast at night.

(iv) x = 2 and x = 3 are the roots of the equation 3x2 - x - 10 = 0.

Q2 :

Identify the quantifier in the following statements and write the negation of the statements.

(i) There exists a number which is equal to its square.

(ii) For every real number x, x is less than x + 1.

(iii) There exists a capital for every state in India.

Q3 :

Check whether the following pair of statements is negation of each other. Give reasons for the answer.

(i) x + y = y + x is true for every real numbers x and y.

(ii) There exists real number x and y for which x + y = y + x.

Q4 :

State whether the â€œOrâ€ used in the following statements is â€œexclusive â€œorâ€ inclusive. Give reasons for your answer.

(i) Sun rises or Moon sets.

(ii) To apply for a driving licence, you should have a ration card or a passport.

(iii) All integers are positive or negative.

### Exercise 14.4 : Solutions of Questions on Page Number : 338

Q1 :

Rewrite the following statement with â€œif-thenâ€ in five different ways conveying the same meaning.

If a natural number is odd, then its square is also odd.

Q2 :

Write the contrapositive and converse of the following statements.

(i) If x is a prime number, then x is odd.

(ii) It the two lines are parallel, then they do not intersect in the same plane.

(iii) Something is cold implies that it has low temperature.

(iv) You cannot comprehend geometry if you do not know how to reason deductively.

(v) x is an even number implies that x is divisible by 4

Q3 :

Write each of the following statement in the form â€œif-thenâ€.

(i) You get a job implies that your credentials are good.

(ii) The Banana trees will bloom if it stays warm for a month.

(iii) A quadrilateral is a parallelogram if its diagonals bisect each other.

(iv) To get A+ in the class, it is necessary that you do the exercises of the book.

Q4 :

Given statements in (a) and (b). Identify the statements given below as contrapositive or converse of each other.

(a) If you live in Delhi, then you have winter clothes.

(i) If you do not have winter clothes, then you do not live in Delhi.

(ii) If you have winter clothes, then you live in Delhi.

(b) If a quadrilateral is a parallelogram, then its diagonals bisect each other.

(i) If the diagonals of a quadrilateral do not bisect each other, then the quadrilateral is not a parallelogram.

(ii) If the diagonals of a quadrilateral bisect each other, then it is a parallelogram.

### Exercise 14.5 : Solutions of Questions on Page Number : 342

Q1 :

Show that the statement

p: â€œIf x is a real number such that x3 + 4x = 0, then x is 0â€ is true by

(i) direct method

(iii) method of contrapositive

Q2 :

Show that the statement â€œFor any real numbers a and b, a2 = b2 implies that a = bâ€ is not true by giving a counter-example.

Q3 :

Show that the following statement is true by the method of contrapositive.

p: If x is an integer and x2is even, then x is also even.

Q4 :

By giving a counter example, show that the following statements are not true.

(i) p: If all the angles of a triangle are equal, then the triangle is an obtuse angled triangle.

(ii) q: The equation x2 - 1 = 0 does not have a root lying between 0 and 2.

Q5 :

Which of the following statements are true and which are false? In each case give a valid reason for saying so.

(i) p: Each radius of a circle is a chord of the circle.

(ii) q: The centre of a circle bisects each chord of the circle.

(iii) r: Circle is a particular case of an ellipse.

(iv) s: If x and y are integers such that x > y, then â€“x < â€“y.

(v) t: is a rational number.

### Exercise Miscellaneous : Solutions of Questions on Page Number : 345

Q1 :

Write the negation of the following statements:

(i) p: For every positive real number x, the number x - 1 is also positive.

(ii) q: All cats scratch.

(iii) r: For every real number x, either x > 1 or x < 1.

(iv) s: There exists a number x such that 0 < x < 1.

Q2 :

State the converse and contrapositive of each of the following statements:

(i) p: A positive integer is prime only if it has no divisors other than 1 and itself.

(ii) q: I go to a beach whenever it is a sunny day.

(iii) r: If it is hot outside, then you feel thirsty.

Q3 :

Write each of the statements in the form â€œif p, then qâ€.

(i) p: It is necessary to have a password to log on to the server.

(ii) q: There is traffic jam whenever it rains.

(iii) r: You can access the website only if you pay a subscription fee.

Q4 :

Re write each of the following statements in the form â€œp if and only if qâ€.

(i) p: If you watch television, then your mind is free and if your mind is free, then you watch television.

(ii) q: For you to get an A grade, it is necessary and sufficient that you do all the homework regularly.

(iii) r: If a quadrilateral is equiangular, then it is a rectangle and if a quadrilateral is a rectangle, then it is equiangular.

Q5 :

Given below are two statements

p:25 is a multiple of 5.

q: 25 is a multiple of 8.

Write the compound statements connecting these two statements with â€œAndâ€ and â€œOrâ€. In both cases check the validity of the compound statement.

Q6 :

Check the validity of the statements given below by the method given against it.

(i) p: The sum of an irrational number and a rational number is irrational (by contradiction method).

(ii) q: If n is a real number with n > 3, then n2 > 9 (by contradiction method).

Q7 :

Write the following statement in five different ways, conveying the same meaning.

p: If triangle is equiangular, then it is an obtuse angled triangle.