اختبار إلكتروني: Conjectures and Counterexamples - Reveal
Exploring Conjectures and Counterexamples in Mathematics. A conjecture is a mathematical statement that appears to be true based on patterns and observations but has not been formally proven. Inductive reasoning is the process of arriving at these conjectures by noticing recurring trends in data or sequences. However, to disprove a conjecture, one only needs to find a single counterexample—a specific case where the statement does not hold true. This worksheet covers identifying patterns, forming conjectures, and finding counterexamples across various mathematical scenarios, including number sequences and geometric properties.
رقم الاختبار976
الصفالصف التاسع المتقدم
المادةرياضيات
الفصلالفصل الثالث
السنة الدراسية2025/2026
عدد الأسئلة24
إجمالي النقاط24
تاريخ الإضافة2026-04-23
الزيارات22
المعلم أو الناشرAmal Salman
اختر إجابة واحدة لكل سؤال. عند الاختيار ستظهر النتيجة فورًا: الأخضر صحيح، والأحمر خطأ، وسيظهر تفسير الإجابة مباشرة إن كان متوفرًا. وبعد آخر سؤال ستظهر الدرجة النهائية تلقائيًا.
Question 1
Points: 1
To fully disprove a conjecture, one needs to find only ONE counterexample.
Explanation
In logic and mathematics, a single counterexample is sufficient to prove that a general statement or conjecture is false.
Question 2
Points: 1
Which number is a counterexample to the following statement? For all numbers a, \(2a + 7 \leq 17\)
Explanation
A counterexample must make the inequality false. If \(a = 6\), then \(2(6) + 7 = 12 + 7 = 19\). Since 19 is not less than or equal to 17, \(a = 6\) is a counterexample.
Question 3
Points: 1
Which of the following conjectures is false?
Explanation
The sum of two odd numbers is always even (e.g., \(1 + 3 = 4\)), making this conjecture false.
Question 4
Points: 1
How many counters would come next?
Explanation
The pattern of dots is 1, 3, 5, 7. Following this arithmetic sequence of odd numbers, the next term is 9.
Question 5
Points: 1
Find the next term in the sequence: A, D, G, J, _____
Explanation
The sequence skips two letters between each term: A (bc) D (ef) G (hi) J (kl) M.
Question 6
Points: 1
Which of the following is the basis for inductive reasoning?
Explanation
Inductive reasoning involves making generalizations based on specific observations or patterns.
Question 7
Points: 1
For a conjecture to be true, it must be true...
Explanation
In mathematics, a conjecture is only considered true if it holds for every possible instance.
Question 8
Points: 1
If an animal is furry, then it is a hamster. What would be an appropriate counterexample?
Explanation
A counterexample must satisfy the 'if' part (be furry) but not the 'then' part (not be a hamster). A cat is furry but not a hamster.
Question 9
Points: 1
Find the next number in the sequence. 1, -1, 2, -2, 3, ___
Explanation
The sequence alternates between positive and negative integers: 1, -1, 2, -2, 3, so the next number is -3.
Question 10
Points: 1
Determine if this conjecture is true. If not, give a counterexample. The difference of two negative numbers is a negative number.
Explanation
Subtracting a negative number is the same as adding its absolute value. In the case of \(-11 - (-13)\), the result is 2, which is positive, disproving the conjecture.
Question 11
Points: 1
If it is an angle, then it is acute. What is an appropriate counterexample?
Explanation
A counterexample must be an angle that is NOT acute. \(120^\circ\) is an obtuse angle, so it disproves the statement.
Question 12
Points: 1
If it is a number, then it is either positive or negative. What is an appropriate counterexample?
Explanation
The number 0 is neither positive nor negative, making it a perfect counterexample to the claim.
Question 13
Points: 1
Used to prove that a conjecture is false.
Explanation
A counterexample is a specific instance that shows a general statement is false.
Question 14
Points: 1
Which is a counterexample to the following statement? If an angle is obtuse, then it is \(125^\circ\).
Explanation
A counterexample must be an obtuse angle (greater than \(90^\circ\) and less than \(180^\circ\)) that is not \(125^\circ\). \(160^\circ\) is obtuse and not \(125^\circ\).
Question 15
Points: 1
Which of the following provide a counterexample to the conjecture, 'If two angles are supplementary, then they are not congruent.'
Explanation
Supplementary angles add up to \(180^\circ\). Two \(90^\circ\) angles are supplementary and they are congruent (equal), which disproves the conjecture.
Question 16
Points: 1
What is a counterexample?
Explanation
This is the definition of a counterexample in the context of logic and mathematical conjectures.
Question 17
Points: 1
Provide a counterexample to the following claim: 'If a number is divisible by 2, then it is divisible by 4.'
Explanation
14 is divisible by 2 (\(14 \div 2 = 7\)) but not divisible by 4 (\(14 \div 4 = 3.5\)), making it a counterexample.
Question 18
Points: 1
Find the pattern to solve the sequence 2, 4, 7, 11...
Explanation
The difference between terms increases by 1 each time: \(2+2=4\), \(4+3=7\), \(7+4=11\).
Question 19
Points: 1
A concluding statement reached using inductive reasoning is called a _______
Explanation
A conjecture is the conclusion formed through inductive reasoning before it is proven.
Question 20
Points: 1
What is a conjecture?
Explanation
A conjecture is an unproven statement that is thought to be true based on observing patterns.
Question 21
Points: 1
The type of reasoning where a person makes conclusions based on observations and patterns is called...
Explanation
Inductive reasoning is specifically defined as deriving general conclusions from specific observed patterns.
Question 22
Points: 1
How would you describe this pattern's rule? 65, 62, 59, 56, 53, 50
Explanation
Each term is 3 less than the previous term (\(65-3=62\), \(62-3=59\), etc.).
Question 23
Points: 1
What are the missing numbers in this pattern? 23, 33, ___, 53, ___, 73
Explanation
The pattern increases by 10 each time: \(23, 33, 43, 53, 63, 73\).
Question 24
Points: 1
Inductive Reasoning means...
Explanation
Inductive reasoning is the scientific and mathematical process of using observed data and patterns to form a general conclusion or conjecture.
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