Algebraic identities

Correct answer: A. 0

Using \( (a+b)^2=a^2+2ab+b^2 \), with \(a=x\) and \(b=3y\), we get \( (x+3y)^2=x^2+6xy+9y^2 \). Thus, the \(x^2\), \(6xy\), and \(9y^2\) terms all cancel in the given expression, leaving \(0\). \(6xy\) is only the middle term in the expansion, not the final value. Exam tip: always check the middle term \(2ab\) when expanding a square.

Correct answer: A. \(x^2+10xy+25y^2\)

The identity \((a+b)^2=a^2+2ab+b^2\) applies here. Taking \(a=x\) and \(b=5y\), the middle term is \(2\times x\times5y=10xy\). Option C represents a square of a difference. Exam tip: always check both the sign and coefficient of the middle term.

Correct answer: A. Terms with odd powers of \(b\) have negative coefficients.

In \((a-b)^3=a^3-3a^2b+3ab^2-b^3\), the terms \(-3a^2b\) and \(-b^3\) contain odd powers of \(b\), so their coefficients are negative. The \(b^2\) term is positive. Exam tip: track signs by parity.

Correct answer: A. (x^3-12x^2+48x-64)

In the cube of a difference, signs follow (+), (-), (+), (-). Exam tip: check both (4^2) and (4^3).

Correct answer: A. (27x^3+108x^2+144x+64)

( (3x)^3=27x^3 ), (3(3x)^2\cdot4=108x^2), and (3(3x)\cdot16=144x). Exam tip: check the power of every term.

Correct answer: A. \(x^2-49\)

\(x^2-49=x^2-7^2\), so it factors as \((x+7)(x-7)\). The factors differ only by sign. \(x^2+49\) is a sum of squares. Exam tip: first check whether both terms are perfect squares.

Correct answer: A. ( (6p-5q)(36p^2+30pq+25q^2) )

This is again a difference of cubes. The first term is 216p³=(6p)³, and the second is 125q³=(5q)³. Applying \(a^3-b^3=(a-b)(a^2+ab+b^2)\) with a=6p and b=5q gives \( (6p-5q)(36p^2+30pq+25q^2)\). Therefore option A is correct.

To verify, the first factor supplies the difference of the cube roots, while the second factor contains the squares and positive cross-product: \( (6p)^2=36p^2\), \( (6p)(5q)=30pq\), and \( (5q)^2=25q^2\). The second factor must have all positive signs for a difference of cubes. Option B has the wrong signs, option C uses incorrect cube roots, and option D is the cube of a binomial rather than the difference-of-cubes identity. The supplied factorisation matches the standard rule exactly.

Correct answer: A. \(x^3-27\)

\(x^3-27=x^3-3^3\), so it is a difference of two perfect cubes. Using \(a^3-b^3=(a-b)(a^2+ab+b^2)\), it factorises as \((x-3)(x^2+3x+9)\). In contrast, \(x^3+27\) is a sum of cubes. Exam tip: recognise \(27=3^3\).

Correct answer: A. \(a^3-b^3\)

The difference-of-cubes identity is \(a^3-b^3=(a-b)(a^2+ab+b^2)\). For \(a^3+b^3\), the first factor is \(a+b\). Exam tip: in the difference of cubes, the middle term of the quadratic factor is positive.

Correct answer: C. \(xyz\)

\((x+y+z)^2=x^2+y^2+z^2+2xy+2yz+2zx\). It contains square terms and products of pairs of variables only; \(xyz\) is a product of three variables, so it is absent. Exam tip: list every pairwise product with coefficient \(2\).

Correct answer: A. \(4a^2-12ab+9b^2\)

\(4a^2-12ab+9b^2=(2a-3b)^2\). Its middle term is \(-2\times2a\times3b=-12ab\). Option D has \(-6ab\), so it is not a perfect square. Exam tip: compare the middle term with twice the product of the square roots of the outer terms.

Correct answer: A. \((7m+3n)(7m-3n)\)

Option A contains conjugate binomials with the same terms and opposite signs. Using \((A+B)(A-B)=A^2-B^2\), it becomes \((7m)^2-(3n)^2\). The other options are perfect squares. Exam tip: look for opposite signs in conjugate factors.

Correct answer: A. (x^2+24x-25)

The difference of the first two squares is (24x), and ( (x+5)(x-5)=x^2-25 ). Exam tip: combine identities while solving.

Correct answer: A. (24ab^2)

The required term is the part containing exactly one a and two b factors. In a cube such as \( (u+v)^3\), the mixed term containing \(uv^2\) is \(3uv^2\). Here the two cubes have opposite signs in their middle terms, and the entire second cube is subtracted. Consequently, the two contributions add rather than cancel.

For the first cube, take \(u=4a\) and \(v=b\). Its required term is \(3(4a)b^2=12ab^2\). In \( (4a-b)^3\), the corresponding term is \(-12ab^2\). Since the expression subtracts this whole cube, the contribution becomes \(-(-12ab^2)=+12ab^2\). Thus the total is \(12ab^2+12ab^2=24ab^2\). Option A is correct. Option B counts only one cube and therefore gives half the result.

Correct answer: A. check complete terms, signs, powers, and middle term separately

Most mistakes happen due to incomplete terms or wrong signs. Exam tip: identify the terms before applying the formula.

Correct answer: C. (16)

The first expansion is (x^2+12x+36) and the second is (x^2+12x+20). Exam tip: cancel like terms and compare constants.

Correct answer: B. (322)

The direct answer is option B, 322. Let \\(a=x\\) and \\(b=\\frac{1}{x}\\). We are given \\(a+b=7\\). Also, \\(ab=x\\cdot\\frac{1}{x}=1\\), provided \\(x\\neq0\\). Use the identity \\(a^3+b^3=(a+b)^3-3ab(a+b)\\). Substitution gives \\(7^3-3(1)(7)=343-21=322\\). Thus option B is correct. Option A, 336, does not subtract the required \\(3ab(a+b)\\) term. Option B follows the identity exactly. Option C, 343, is only \\(7^3\\); it forgets that \\(a^3+b^3\\) is not simply \\((a+b)^3\\). Option D, 364, results from an incorrect operation and is larger than the correct result. Do not try to find x itself; the identity gives the requested expression directly. Memory cue: if \\(x+1/x=7\\), then the cubes are \\(7^3-3\\times7\\) because the product is 1.

Correct answer: D. ( (3a+2b)(9a^2-6ab+4b^2) )

This is ((3a)^3+(2b)^3), so the sum of cubes identity applies. Exam tip: the middle term inside the second factor is negative.

Correct answer: A. (41)

The direct answer is option A: 41. Use the identity \(a^2+b^2+c^2=(a+b+c)^2-2(ab+bc+ca)\). Substitute the given values step by step: \((a+b+c)^2=9^2=81\), and \(2(ab+bc+ca)=2\cdot20=40\). Therefore \(a^2+b^2+c^2=81-40=41\). Option A is correct. Option B, 49, could result from an incorrect subtraction or from ignoring the factor 2. Option C, 61, is not obtained from the identity and has no valid calculation here. Option D, 81, is only \((a+b+c)^2\); it forgets to subtract the cross-product terms. The identity comes from expanding \((a+b+c)^2\), which contains the three squares and twice the pairwise products. Memory cue: square of sum equals sum of squares plus twice the pair products.

Correct answer: A. (2x^3+24xy^2)

The correct answer is option A, \\(2x^3+24xy^2\\). Use \\( (a+b)^3+(a-b)^3=2a^3+6ab^2\\). Put \\(a=x\\) and \\(b=2y\\). Then \\(2a^3=2x^3\\), while \\(6ab^2=6\cdot x\cdot(2y)^2=6x\cdot4y^2=24xy^2\\). The terms containing an odd power of \\(b\\) cancel. A is therefore correct. B has only half of the second term. C incorrectly keeps a cube term in \\(y\\), although the two opposite cubes cancel. D contains terms of the wrong types, such as \\(x^2y\\), which do not remain after addition. A useful check is to expand both cubes: the \\(x^2y\\) and \\(y^3\\) terms cancel, leaving only \\(2x^3+24xy^2\\).

Correct answer: A. \(a^3-b^3=(a-b)(a^2+ab+b^2)\)

Option A is correct because expanding \((a-b)(a^2+ab+b^2)\) cancels the middle terms and gives \(a^3-b^3\). Option B is for a sum of cubes. Exam tip: when powers are 3, check cube identities first.

Correct answer: C. \(a^3-b^3=(a-b)(a^2+ab+b^2)\)

The difference-of-cubes identity is \(a^3-b^3=(a-b)(a^2+ab+b^2)\). On multiplying, the middle terms \(-a^2b\) and \(+a^2b\) cancel. Exam tip: a difference of cubes always begins with \(a-b\).

Correct answer: A. (4y(2x+3))

The direct answer is option A, \\(4y(2x+3)\\). Let \\(P=2x+3\\). The expression becomes \\((P+y)^2-(P-y)^2\\). Apply \\((A+B)^2-(A-B)^2=4AB\\): here \\(A=P\\) and \\(B=y\\), so the result is \\(4Py=4y(2x+3)\\). Expanding also confirms it: the first square has the middle term \\(4y(2x+3)\\), while the second has \\(-4y(2x+3)\\); subtraction leaves \\(8y(2x+3)\\)? More carefully, their difference is twice the middle-term difference, giving \\(4y(2x+3)\\) by the identity. Option A is correct. Option B is half the required value. Option C incorrectly treats the expression like a simple difference of unrelated squares and loses the cross terms. Option D, \\(8xy+6y\\), equals \\(2y(4x+3)\\), not the required expansion \\(8xy+12y\\). Memory cue: in \\((A+B)^2-(A-B)^2\\), answer is always \\(4AB\\).