Irrational numbers

Correct answer: B. (99)

Explanation: Step 1: The square root of a perfect square is rational. Step 2: (99) is not a perfect square, so (\sqrt{99}) is irrational. Step 3: In such questions, first separate the perfect squares.

Correct answer: B. (\sqrt{6}+5)

Explanation: Step 1: (\sqrt{6}) is irrational because (6) is not a perfect square. Step 2: Adding the rational number (5) to an irrational number keeps the result irrational. Step 3: Simplify options before deciding the nature of the number.

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

Explanation: Step 1: (\sqrt{147}=7\sqrt{3}) and (\sqrt{75}=5\sqrt{3}). Step 2: (7\sqrt{3}-5\sqrt{3}=2\sqrt{3}). Step 3: Before subtracting radicals, convert them into like radicals.

Correct answer: B. Irrational

Explanation: Step 1: (x-7=(7+\sqrt{13})-7). Step 2: This leaves (\sqrt{13}), which is irrational. Step 3: In such expressions, first subtract the matching rational part.

Correct answer: D. (\sqrt{63})

Explanation: Step 1: Terminating decimals, fractions, and recurring decimals are rational. Step 2: (\sqrt{63}=3\sqrt{7}), and (\sqrt{7}) is irrational. Step 3: Simplifying the square-root option is a good way to check it.

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

Explanation: Step 1: (\sqrt{28}=2\sqrt{7}), (\sqrt{63}=3\sqrt{7}), and (\sqrt{175}=5\sqrt{7}). Step 2: The sum is (2\sqrt{7}+3\sqrt{7}+5\sqrt{7}=10\sqrt{7}). Step 3: Once radicals become like terms, add only the coefficients.

Correct answer: C. The difference of a rational and an irrational number is irrational

Explanation: Step 1: Subtracting an irrational number from a rational number leaves an irrational part. Step 2: For example, (5-\sqrt{2}) is irrational. Step 3: For always-type statements, checking counterexamples is useful.

Correct answer: B. (3)

Explanation: Step 1: First write (\sqrt{9}=3). Step 2: (\frac{9}{\sqrt{9}}=\frac{9}{3}=3). Step 3: Rationalisation is not always needed; first evaluate square roots of perfect squares.

Correct answer: B. (\sqrt{k}\times\sqrt{k})

Explanation: Step 1: Multiplying a square root by itself gives the number inside. Step 2: (\sqrt{k}\times\sqrt{k}=k), which is rational if (k) is an integer. Step 3: The square of an irrational square root can be rational.

Correct answer: B. (\sqrt{130})

Explanation: Step 1: Since (121<130<144), (11<\sqrt{130}<12). Step 2: (130) is not a perfect square, so (\sqrt{130}) is irrational. Step 3: In interval questions, use nearby perfect squares to set the range.

Correct answer: C. (9\sqrt{5})

Explanation: Step 1: (\sqrt{245}=7\sqrt{5}), (\sqrt{180}=6\sqrt{5}), and (\sqrt{80}=4\sqrt{5}). Step 2: (7\sqrt{5}+6\sqrt{5}-4\sqrt{5}=9\sqrt{5}). Step 3: Before addition or subtraction, write all radicals in like form.

Correct answer: C. Irrational

Explanation: Step 1: This decimal has no fixed block of digits repeating again and again. Step 2: It is non-terminating and non-recurring, so it is irrational. Step 3: While deciding the nature of a decimal, look for a clear repeating rule.

Correct answer: A. (\sqrt{11},-\sqrt{11})

Explanation: Step 1: (\sqrt{11}) and (-\sqrt{11}) are both irrational. Step 2: Their sum is (0), which is rational. Step 3: Opposite irrational terms can give a rational sum.

Correct answer: A. (4\sqrt{7}+7)

Explanation: Step 1: Apply distribution: (\sqrt{7}\times4+\sqrt{7}\times\sqrt{7}). Step 2: This becomes (4\sqrt{7}+7). Step 3: In such multiplication, remember (\sqrt{7}\times\sqrt{7}=7).

Correct answer: A. (\sqrt{75})

Explanation: Step 1: (5\sqrt{3}=\sqrt{25}\sqrt{3}). Step 2: This equals (\sqrt{75}). Step 3: When moving an outside coefficient inside the root, multiply by its square.

Correct answer: A. (9)

Explanation: Step 1: This is of the form \((a+b)(a-b)=a^2-b^2\). Step 2: \(4^2-(\sqrt{7})^2=16-7=9\). Step 3: In conjugate multiplication, directly use the difference of squares.

Correct answer: C. (\sqrt{225}-8)

Explanation: Step 1: (\sqrt{225}=15). Step 2: (\sqrt{225}-8=7), which is rational. Step 3: When asked for not irrational, carefully check perfect-square options.

Correct answer: A. (-4)

Explanation: Step 1: (ab=(\sqrt{5}+3)(\sqrt{5}-3)). Step 2: Using difference of squares, ((\sqrt{5})^2-3^2=5-9=-4). Step 3: Recognising conjugate form makes the calculation shorter.

Correct answer: A. (4-\sqrt{15})

Explanation: Step 1: The conjugate of (4+\sqrt{15}) is (4-\sqrt{15}). Step 2: (\frac{1}{4+\sqrt{15}}\times\frac{4-\sqrt{15}}{4-\sqrt{15}}=\frac{4-\sqrt{15}}{16-15}=4-\sqrt{15}). Step 3: Use the conjugate of the denominator for rationalisation.

Correct answer: C. The product of two irrational numbers is always irrational

Explanation: Step 1: (\sqrt{3}) and (\sqrt{3}) are both irrational. Step 2: Their product is (3), which is rational. Step 3: Test always-type statements with a counterexample.

Correct answer: A. (15\sqrt{11})

Explanation: Step 1: (\sqrt{44}=2\sqrt{11}), (\sqrt{99}=3\sqrt{11}), and (\sqrt{176}=4\sqrt{11}). Step 2: The sum should be (9\sqrt{11}); the listed options do not contain it. Step 3: If options miss the correct value, the question should be revised.

Correct answer: A. (7+4\sqrt{7})

Explanation: Step 1: (x^2=(\sqrt{7})^2=7). Step 2: (4x=4\sqrt{7}), so (x^2+4x=7+4\sqrt{7}). Step 3: Simplify the square and the multiplication separately.

Correct answer: B. (150)

Explanation: Step 1: (12<\sqrt{n}<13) means (144<n<169). Step 2: (150) lies in this interval and is not a perfect square, so (\sqrt{150}) is irrational. Step 3: Square both bounds to handle square-root ranges.

Correct answer: C. (0.818181\ldots)

Explanation: Step 1: A recurring decimal is rational. Step 2: In (0.818181\ldots), the block (81) repeats. Step 3: Identifying the repeating block in a decimal is important.

Correct answer: A. \(15+4\sqrt{11}\)

Explanation: Step 1: Use \((a+b)^2=a^2+2ab+b^2\). Step 2: \((\sqrt{11})^2+2\sqrt{11}\times2+2^2=11+4\sqrt{11}+4=15+4\sqrt{11}\). Step 3: Forgetting the middle term (2ab) is a common mistake.