Real Numbers

Correct answer: B. It is irrational because the lengths of the groups of zeros keep changing, so there is no fixed repeating block.

Explanation: Option B is correct. The decimal expansion of a rational number is either terminating or non-terminating recurring, meaning that a fixed block of digits repeats. Here, the number of zeros between successive 1s keeps increasing, so no fixed repeating block exists. Option D reaches the right classification for this number but gives a wrong reason: a non-terminating recurring decimal such as \(0.333\ldots\) is rational. Exam tip: For a non-terminating decimal to be rational, its repeating block must have a fixed length.

Correct answer: A. ( \frac{4\sqrt{3}-3\sqrt{5}}{3} )

Explanation: The direct answer is option A: \((4\sqrt{3}-3\sqrt{5})/3\). Start with \(1/(4\sqrt{3}+3\sqrt{5})\). Its conjugate is \(4\sqrt{3}-3\sqrt{5}\). Multiply top and bottom by that conjugate. The denominator is \((4\sqrt{3})^2-(3\sqrt{5})^2=48-45=3\), so the result is \((4\sqrt{3}-3\sqrt{5})/3\). Option A is exact. Option B is its negative, because the numerator terms are reversed. Option C uses the original plus expression and an incorrect denominator 93. Option D omits the denominator 3, so it is three times too large. The key idea is difference of squares: \((p+q)(p-q)=p^2-q^2\).

Correct answer: A. ( \sqrt{13}+\sqrt{8} )

Explanation: Multiplying by the conjugate gives denominator (13-8=5). The numerator also has (5), so the answer is ( \sqrt{13}+\sqrt{8} ).

Correct answer: A. 46/9

Explanation: Apply the conjugate-denominator method. Let a=4+√7 and b=4−√7. The sum a/b+b/a has common denominator ab. Calculate ab=(4+√7)(4−√7)=4²−(√7)²=16−7=9. The numerator is a²+b²=(4+√7)²+(4−√7)². Expanding gives (16+8√7+7)+(16−8√7+7)=46, because the opposite cross-terms cancel. Therefore the value is 46/9, so option A is correct. Option B is exactly half the correct numerator and suggests that one squared expression was omitted. Option C comes from an incorrect expansion of the radical terms. Option D incorrectly treats the two fractions as if their sum were simply 2, ignoring their non-unit denominators. Direct substitution also confirms that both original denominators are nonzero.

Correct answer: A. \(4\sqrt{14}\)

Explanation: Use the identity \((a+b)^2-(a-b)^2=4ab\). Here, \(a=\sqrt{7}\) and \(b=\sqrt{2}\), so the value is \(4\times\sqrt{7}\times\sqrt{2}=4\sqrt{14}\). \(2\sqrt{14}\) results from incorrectly using 2 instead of 4. Exam tip: Apply this identity directly instead of expanding both squares.

Correct answer: A. (5)

Explanation: ( \sqrt{20}=2\sqrt{5} ) and ( \sqrt{45}=3\sqrt{5} ). Dividing (5\sqrt{5}) by ( \sqrt{5} ) gives (5).

Correct answer: A. \(\sqrt{2},\ -\sqrt{2}\)

Explanation: Both \(\sqrt{2}\) and \(-\sqrt{2}\) are irrational, but their sum is \(\sqrt{2}+(-\sqrt{2})=0\), which is rational. Hence, the claim that the sum of two irrational numbers is always irrational is false. In option B, \(\sqrt{2}+\sqrt{3}\) is still irrational, so it is not a counterexample. Exam tip: To disprove a statement containing words such as “always,” it is enough to find one valid counterexample.

Correct answer: A. ( \sqrt{10}-2 )

Explanation: Direct answer: option A, \(\sqrt{10}-2\). Start by recognising a square: \((\sqrt{10}-2)^2=10-4\sqrt{10}+4=14-4\sqrt{10}\). Hence the given radical is \(\sqrt{(\sqrt{10}-2)^2}=|\sqrt{10}-2|\). Since \(\sqrt{10}>2\), this absolute value equals \(\sqrt{10}-2\), which is positive. Option A is correct. Option B gives the square of \(\sqrt{10}+2\), namely \(14+4\sqrt{10}\), not the given expression. Option C is negative because \(2<\sqrt{10}\), so it cannot be the positive principal root. Option D does not square to the given expression. Remember: \(\sqrt{a^2}=|a|\), then check the sign.

Correct answer: D. \(0.101001000100001\ldots\)

Explanation: In \(0.101001000100001\ldots\), the number of zeros between successive 1s keeps increasing. Hence, no fixed block of digits repeats, so its decimal expansion is non-terminating and non-recurring; therefore, it is irrational. \(\frac{7}{125}\) has a terminating decimal expansion, while \(0.\overline{27}\) is recurring. Also, \(\sqrt{81}=9\), which is rational. Exam tip: A non-terminating decimal is irrational only when it does not repeat a fixed pattern.

Correct answer: B. The decimal expansion terminates if and only if \(q=2^m5^n\), where \(m,n\) are non-negative integers.

Explanation: When \(p/q\) is in lowest terms, its decimal expansion terminates exactly when the denominator \(q\) has only 2 and 5 as prime factors, that is, \(q=2^m5^n\). If the denominator has any other prime factor, the decimal expansion is non-terminating recurring, but the number is still rational. Hence options C and D are incorrect. Exam tip: Always reduce a fraction to lowest terms before checking its decimal expansion.

Correct answer: A. ( -\sqrt{80} )

Explanation: ( \sqrt{80}\approx8.94 ), so ( -\sqrt{80}\approx-8.94 ). It is greater than ( -9 ) because it is closer to zero.

Correct answer: A. Terminating decimal

Explanation: ( \frac{96}{384}=\frac{1}{4} ) and (4=2^2). A decimal terminates when the denominator has only (2) and (5) as prime factors.

Correct answer: B. Non-terminating repeating decimal

Explanation: The direct answer is option B: non-terminating repeating decimal. Simplify the fraction by dividing numerator and denominator by 12: \(132/360=11/30\). A rational fraction has a terminating decimal only when, in lowest terms, its denominator has no prime factors except 2 and 5. Here \(30=2\times3\times5\), so the factor 3 remains. Therefore the decimal does not end; it repeats: \(11/30=0.3666\ldots\). Option A is wrong because a factor 3 remains. Option B is correct. Option C describes a non-repeating irrational-type decimal, but this is a rational fraction. Option D is wrong because every ratio of integers with a nonzero denominator is rational. Cancel first, then inspect the denominator.

Correct answer: B. Rational real number

Explanation: Direct answer: option B, rational real number. The digits 81 repeat without stopping, so the number is \(7.818181\ldots=7.\overline{81}\). Every terminating or recurring decimal can be represented as a ratio of integers, so it is rational. Option A is wrong because irrational numbers have non-terminating, non-repeating decimals. Option B is correct because the repeating block is visible. Option C is wrong: an integer is a whole number such as 7 or 8, whereas this number has a fractional part and lies between them. Option D is wrong because the decimal is meaningful and defined. A useful test is to look for a fixed repeating block; repetition proves rationality.

Correct answer: A. \(10\sqrt{7}\)

Explanation: \(\sqrt{28}=\sqrt{4\times7}=2\sqrt{7}\), \(\sqrt{63}=\sqrt{9\times7}=3\sqrt{7}\), and \(\sqrt{112}=\sqrt{16\times7}=4\sqrt{7}\). Hence, the sum is \(\sqrt{7}+2\sqrt{7}+3\sqrt{7}+4\sqrt{7}=(1+2+3+4)\sqrt{7}=10\sqrt{7}\). An option such as \(8\sqrt{7}\) can result from using an incorrect coefficient for one term. Exam tip: extract perfect-square factors from surds before adding like surd terms.

Correct answer: A. \(2\sqrt{6}\)

Explanation: Use a common denominator for the two fractions. The denominator product is \\( (\sqrt7-\sqrt6)(\sqrt7+\sqrt6)=7-6=1\\), by the difference-of-squares identity. The combined numerator is \\(\sqrt7+\sqrt6-(\sqrt7-\sqrt6)=2\sqrt6\\). Since the denominator equals 1, the whole expression is \\(2\sqrt6\\). Therefore option A is correct.

The subtraction sign is important: subtracting the second numerator changes both signs inside its parentheses, so the \\(\sqrt7\\) terms cancel and the two \\(\sqrt6\\) terms add. Both denominators are nonzero because \\(\sqrt7\ne\sqrt6\\). A numerical check gives a value near 4.90, matching \\(2\sqrt6\\). Thus the supplied answer A is valid and follows directly from conjugates and the difference-of-squares formula.

Correct answer: A. \( \frac{5}{2} \)

Explanation: Use the addition rule for two fractions and the conjugate identity \((a-b)(a+b)=a^2-b^2\). The common denominator is \((5-\sqrt{21})(5+\sqrt{21})=25-21=4\). The numerator is \((5+\sqrt{21})+(5-\sqrt{21})=10\), because the irrational terms cancel. Therefore the expression equals \(10/4=5/2\), so option A is correct. Option B incorrectly uses 21 as the effective denominator, option C ignores the cancellation, and option D misses the factor of 2 produced by the two fractions. The denominator is nonzero, so the calculation is valid.

Correct answer: A. 7

Explanation: This is a product of conjugates in the form \((a+b)(a-b)=a^2-b^2\). Here, \(a=\sqrt{10}\) and \(b=\sqrt{3}\), so the value is \(10-3=7\). \(2\sqrt{30}\) relates to the middle terms in expansion, but these terms cancel for conjugates. Exam tip: Whenever you see \((x+y)(x-y)\), apply the difference of squares formula directly.

Correct answer: A. (0)

Explanation: They become (2\sqrt{5},6\sqrt{5},4\sqrt{5},3\sqrt{5}). Since (2+6-4-3=1), the result should be ( \sqrt{5} ).

Correct answer: A. (98)

Explanation: (a^2=7+2\sqrt{10}) and ( \frac{1}{a^2} ) is not simply (7-2\sqrt{10}) because (a(\sqrt{5}-\sqrt{2})=3). Check options carefully using reciprocal rules.

Correct answer: A. (3+\frac{5\sqrt{2}}{2})

Explanation: (x-3=\sqrt{8}=2\sqrt{2}), so ( \frac{1}{x-3}=\frac{\sqrt{2}}{4} ). The total should be (3+2\sqrt{2}+\frac{\sqrt{2}}{4}=3+\frac{9\sqrt{2}}{4}).

Correct answer: C. \(|a-b|\)

Explanation: \(\sqrt{x^2}=|x|\) because the principal square root is always non-negative. Taking \(x=a-b\), we get \(\sqrt{(a-b)^2}=|a-b|\). The expression \(a-b\) is correct only when \(a\ge b\); if \(a<b\), it is negative. Exam tip: whenever a squared expression comes out of a square root, remember to use absolute value signs.

Correct answer: A. \(-18\)

Explanation: \(\sqrt{x^2}=|x|\), because the principal square root is always non-negative. Thus, for \(x=-9\), \(\sqrt{x^2}=|-9|=9\) and \(3x=3(-9)=-27\). Therefore, \(\sqrt{x^2}+3x=9-27=-18\). Taking \(\sqrt{x^2}\) directly as \(x\) would give \(-36\), which is incorrect. Exam tip: always rewrite \(\sqrt{x^2}\) as \(|x|\).

Correct answer: A. ( \sqrt{27}-4 )

Explanation: ( \sqrt{27}>4 ) because (27>16). So (4-\sqrt{27}) is negative and the absolute value is ( \sqrt{27}-4 ).

Correct answer: A. \(\sqrt{2}\times\sqrt{2}=2\)

Explanation: \(\sqrt{2}\) is irrational, but \(\sqrt{2}\times\sqrt{2}=2\), which is rational. Therefore, the product of two irrational numbers is not always irrational. In option C, \(\sqrt{6}\) is irrational, so it does not disprove the claim. Exam tip: To disprove an “always” statement, one valid counterexample is enough.