Solving quadratics and applications

106 questions · 21 question types identified

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Quadratic in higher integer powers

A question is this type if and only if it requires solving an equation of the form ax^(2n) + bx^n + c = 0 where n >= 2 is a positive integer (e.g. x^4, x^6), treated as a quadratic in x^n.

10 Moderate -0.3
9.4% of questions
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Solve the equation \(8x^6 + 7x^3 - 1 = 0\). [5]
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Easiest question Moderate -0.8 »
9 Solve the equation \(y ^ { 2 } - 7 y + 12 = 0\).
Hence solve the equation \(x ^ { 4 } - 7 x ^ { 2 } + 12 = 0\). Section B (36 marks)
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Hardest question Standard +0.3 »
10 Given that \(\mathrm { f } ( x ) = 8 x ^ { 3 } + \frac { 1 } { x ^ { 3 } }\),
  1. find \(\mathrm { f } ^ { \prime \prime } ( x )\),
  2. solve the equation \(\mathrm { f } ( x ) = - 9\).
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Rearranging formula - single step isolation (square/root/fraction)

Rearrange a formula where the target variable appears once and only one or two simple steps are needed (e.g. squaring, taking a root, or multiplying out a single bracket), such as E = ½mv², A = πr²(x+y), V = ⅓πr²h, s = ½at².

10 Easy -1.6
9.4% of questions
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1 Make \(v\) the subject of the formula \(E = \frac { 1 } { 2 } m v ^ { 2 }\).
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Easiest question Easy -2.0 »
2 Make \(t\) the subject of the formula \(s = \frac { 1 } { 2 } a t ^ { 2 }\).
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Hardest question Moderate -0.8 »
3 Rearrange the following formula to make \(r\) the subject, where \(r > 0\). $$V = \frac { 1 } { 3 } \pi r ^ { 2 } ( a + b )$$
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Substitution to solve disguised quadratic

A question is this type if and only if it requires an explicit substitution (e.g. u = x^(1/3), y = 2^x, u = (3x-2)^2) to reduce a non-standard equation into a solvable quadratic or factorable form.

8 Moderate -0.4
7.5% of questions
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2 By using a suitable substitution, solve the equation $$( 2 x - 3 ) ^ { 2 } - \frac { 4 } { ( 2 x - 3 ) ^ { 2 } } - 3 = 0$$
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Finding quadratic constants from algebraic conditions

Determine unknown constants in a quadratic (or polynomial) expression from purely algebraic conditions such as a known minimum/stationary point, specific function values, or given roots, with no real-world context or graph of a trajectory involved.

8 Moderate -0.6
7.5% of questions
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You are given that \(f(x) = x^2 + kx + c\). Given also that \(f(2) = 0\) and \(f(-3) = 35\), find the values of the constants \(k\) and \(c\). [4]
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Optimisation via quadratic model

A question is this type if and only if a real-world quantity (profit, area, fencing) is modelled by a quadratic function and the question asks for the maximum or minimum value and the corresponding variable value, using completing the square or differentiation.

7 Moderate -0.4
6.6% of questions
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7 \includegraphics[max width=\textwidth, alt={}, center]{56d376c5-b91f-488d-89e2-18edcb14052d-3_534_895_255_625} The diagram shows the dimensions in metres of an L-shaped garden. The perimeter of the garden is 48 m .
  1. Find an expression for \(y\) in terms of \(x\).
  2. Given that the area of the garden is \(A \mathrm {~m} ^ { 2 }\), show that \(A = 48 x - 8 x ^ { 2 }\).
  3. Given that \(x\) can vary, find the maximum area of the garden, showing that this is a maximum value rather than a minimum value.
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Quadratic inequality solving

A question is this type if and only if it requires solving a quadratic inequality (e.g. 2x^2 - x - 3 > 0) and expressing the solution as a set of values or intervals, possibly after sketching the curve.

6 Moderate -0.7
5.7% of questions
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2
  1. Factorise \(3 x ^ { 2 } - 19 x - 14\).
  2. Solve the inequality \(3 x ^ { 2 } - 19 x - 14 < 0\).
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Quadratic with surd roots, exact form

A question is this type if and only if it requires solving a quadratic equation and expressing the roots exactly in surd form (e.g. p ± q√r), typically using the quadratic formula.

6 Moderate -0.3
5.7% of questions
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5 Justify the statement in line 87 that $$\frac { 1 } { \phi } = \frac { \sqrt { 5 } - 1 } { 2 }$$
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Quadratic in x^(1/2) - substitution u = √x

Solve equations where substituting u = x^(1/2) (or u = √x) reduces the equation to a standard quadratic in u, e.g. 4x - 11x^(1/2) + 6 = 0, x - 6√x + 4 = 0, 2x - 7x^(1/2) + 3 = 0.

6 Standard +0.2
5.7% of questions
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4 Solve the equation \(2 x - 7 x ^ { \frac { 1 } { 2 } } + 3 = 0\).
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Completing the square, form and properties

A question is this type if and only if it asks to express a quadratic in completed square form a(x+p)^2 + q, and/or use that form to state the vertex, minimum/maximum value, or solve the equation.

5 Moderate -0.5
4.7% of questions
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8
  1. Express \(2 x ^ { 2 } + 5 x\) in the form \(2 ( x + p ) ^ { 2 } + q\).
  2. State the coordinates of the minimum point of the curve \(y = 2 x ^ { 2 } + 5 x\).
  3. State the equation of the normal to the curve at its minimum point.
  4. Solve the inequality \(2 x ^ { 2 } + 5 x > 0\).
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Solving simple rational equations

A question is this type if and only if it requires solving an equation where the unknown appears in a denominator (e.g. (3x+1)/(2x) = 4), leading to a linear or simple quadratic after clearing fractions.

5 Easy -1.2
4.7% of questions
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Solve the equation \(\frac{3x + 1}{2x} = 4\). [3]
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Rearranging formula - variable appears multiple times

Rearrange a formula to make a specified variable the subject where the variable appears more than once, requiring collecting terms and factorising (e.g. y + 5 = x(y + 2), 3(a+4) = ac + 5f, 4h + 5 = 9a - ha²).

5 Moderate -0.7
4.7% of questions
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3 Make \(a\) the subject of the equation $$2 a + 5 c = a f + 7 c$$
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Algebraic identity, find constants

A question is this type if and only if it presents a polynomial identity (e.g. 5x^2 + px - 8 ≡ q(x-1)^2 + r) and asks for the values of the unknown constants by expanding and comparing coefficients.

4 Moderate -0.8
3.8% of questions
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1 Find the values of \(A , B\) and \(C\) in the identity \(4 x ^ { 2 } - 16 x + C \equiv A ( x + B ) ^ { 2 } + 2\).
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Finding quadratic constants from real-world trajectory or context

Determine unknown constants in a quadratic model arising from a real-world context (e.g. trajectory of a ball, exchange rate model) where conditions are given via a described physical situation or graph of a trajectory.

4 Moderate -0.0
3.8% of questions
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  1. A curve has equation \(y = \mathrm { g } ( x )\).
Given that
  • \(\mathrm { g } ( x )\) is a cubic expression in which the coefficient of \(x ^ { 3 }\) is equal to the coefficient of \(x\)
  • the curve with equation \(y = \mathrm { g } ( x )\) passes through the origin
  • the curve with equation \(y = \mathrm { g } ( x )\) has a stationary point at \(( 2,9 )\)
    1. find \(\mathrm { g } ( x )\),
    2. prove that the stationary point at \(( 2,9 )\) is a maximum.
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Quadratic in negative or reciprocal fractional powers

Solve equations where the variable appears with negative or reciprocal exponents (e.g. 1/x², 1/x⁴, x^(1/3), x^(2/3)) that form a quadratic structure upon substitution.

4 Moderate -0.1
3.8% of questions
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3 Solve the equation \(3 x ^ { \frac { 2 } { 3 } } + x ^ { \frac { 1 } { 3 } } - 2 = 0\).
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Rearranging formula - multi-step isolation with nested operations

Rearrange a formula where the target variable appears once but is embedded in nested operations requiring multiple steps to isolate, such as T = 2π√(l/g), c = √((a+b)/2), a = (√y - 5)/c, or V = ⅓πr²√(l²-r²).

4 Moderate -0.9
3.8% of questions
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2 Make \(l\) the subject of the formula \(T = 2 \pi \sqrt { \frac { l } { g } }\).
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Geometric area leads to quadratic

A question is this type if and only if a geometric context (triangle, trapezium, rectangle, L-shape, playground) is used to set up a quadratic equation or expression for area, which is then solved or optimised.

3 Moderate -0.4
2.8% of questions
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9 Fig. 9 shows a trapezium ABCD , with the lengths in centimetres of three of its sides. \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{754d34e4-2f47-48b7-9fbb-6caa7ac21eb7-3_464_878_347_632} \captionsetup{labelformat=empty} \caption{Fig. 9}
\end{figure} This trapezium has area \(140 \mathrm {~cm} ^ { 2 }\).
  1. Show that \(x ^ { 2 } + 2 x - 35 = 0\).
  2. Hence find the length of side AB of the trapezium.
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Quadratic trajectory/projectile model

A question is this type if and only if a physical trajectory (ball, arrow, stone) is modelled by a quadratic function H(x) or h(t), and the question asks to find maximum height, range, or interpret constants of the model.

3 Moderate -0.8
2.8% of questions
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8. \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{580fc9b9-d78c-4a86-91fc-22638cb5186d-20_540_1465_294_301} \captionsetup{labelformat=empty} \caption{Figure 1}
\end{figure} Figure 1 is a graph showing the trajectory of a rugby ball. The height of the ball above the ground, \(H\) metres, has been plotted against the horizontal distance, \(x\) metres, measured from the point where the ball was kicked. The ball travels in a vertical plane. The ball reaches a maximum height of 12 metres and hits the ground at a point 40 metres from where it was kicked.
  1. Find a quadratic equation linking \(H\) with \(x\) that models this situation. The ball passes over the horizontal bar of a set of rugby posts that is perpendicular to the path of the ball. The bar is 3 metres above the ground.
  2. Use your equation to find the greatest horizontal distance of the bar from \(O\).
  3. Give one limitation of the model.
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Curve intersection leads to quadratic

A question is this type if and only if finding the intersection points of two curves (e.g. a polynomial and a line) requires setting up and solving a quadratic or disguised quadratic equation.

2 Standard +0.8
1.9% of questions
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  1. Show that \((x + 1)\) is a factor of \(2x^3 + 3x^2 - 1\) [1]
  2. Solve the equation $$\sqrt{x^2 + 2x + 5} = x + \sqrt{2x + 3}$$ [8]
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Quadratic applied to similar/geometric figures with surds

A question is this type if and only if it involves similar shapes or geometric figures with surd-valued dimensions, requiring multiplication or simplification of surds to find an unknown length.

2 Moderate -0.1
1.9% of questions
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3. \includegraphics[max width=\textwidth, alt={}, center]{4fec0924-d727-4d4f-81e6-918e1ccfedbd-1_330_1230_829_386} The diagram shows the rectangles \(A B C D\) and \(E F G H\) which are similar.
Given that \(A B = ( 3 - \sqrt { 5 } ) \mathrm { cm } , A D = \sqrt { 5 } \mathrm {~cm}\) and \(E F = ( 1 + \sqrt { 5 } ) \mathrm { cm }\), find the length \(E H\) in cm, giving your answer in the form \(a + b \sqrt { 5 }\) where \(a\) and \(b\) are integers.
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Quadratic function range and roots analysis

A question is this type if and only if it asks about the range of a quadratic function, conditions on constants for real/repeated roots, or properties of roots (e.g. discriminant conditions, sum and product of roots).

2 Moderate -0.6
1.9% of questions
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9 The function f is defined for all real values of \(x\) as \(\mathrm { f } ( x ) = c + 8 x - x ^ { 2 }\), where \(c\) is a constant.
  1. Given that the range of f is \(\mathrm { f } ( x ) \leqslant 19\), find the value of \(c\).
  2. Given instead that \(\mathrm { ff } ( 2 ) = 8\), find the possible values of \(c\).
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Quadratic in x^(1/4) - substitution u = x^(1/4)

Solve equations where substituting u = x^(1/4) reduces the equation to a standard quadratic in u, e.g. 3x^(1/2) - 8x^(1/4) + 4 = 0, 2y^(1/2) - 7y^(1/4) + 3 = 0.

2 Standard +0.0
1.9% of questions
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6 Solve the equation \(3 x ^ { \frac { 1 } { 2 } } - 8 x ^ { \frac { 1 } { 4 } } + 4 = 0\).
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