Stationary points and optimisation

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csat-suneung 2018 Q18 4 marks Determine parameters from given extremum conditions
A cubic function $f ( x )$ with leading coefficient 1 and $f ( 1 ) = 0$ satisfies $$\lim _ { x \rightarrow 2 } \frac { f ( x ) } { ( x - 2 ) \left\{ f ^ { \prime } ( x ) \right\} ^ { 2 } } = \frac { 1 } { 4 }$$ Find the value of $f ( 3 )$. [4 points]
(1) 4
(2) 6
(3) 8
(4) 10
(5) 12
csat-suneung 2018 Q20 4 marks Find critical points and classify extrema of a given function
A quartic function $f ( x )$ with leading coefficient 1 satisfies the following conditions. (가) $f ^ { \prime } ( 0 ) = 0 , f ^ { \prime } ( 2 ) = 16$ (나) For some positive number $k$, $f ^ { \prime } ( x ) < 0$ on the two open intervals $( - \infty , 0 ) , ( 0 , k )$. Choose all correct statements from the following. [4 points]
$\langle$Statements$\rangle$ ㄱ. The equation $f ^ { \prime } ( x ) = 0$ has exactly one real root in the open interval $( 0,2 )$. ㄴ. The function $f ( x )$ has a local maximum value. ㄷ. If $f ( 0 ) = 0$, then $f ( x ) \geq - \frac { 1 } { 3 }$ for all real numbers $x$.
(1) ㄱ
(2) ㄴ
(3) ㄱ, ㄷ
(4) ㄴ, ㄷ
(5) ㄱ, ㄴ, ㄷ
csat-suneung 2018 Q29 4 marks Composite or piecewise function extremum analysis
For two real numbers $a$ and $k$, two functions $f ( x )$ and $g ( x )$ are defined as $$\begin{aligned} & f ( x ) = \begin{cases} 0 & ( x \leq a ) \\ ( x - 1 ) ^ { 2 } ( 2 x + 1 ) & ( x > a ) \end{cases} \\ & g ( x ) = \begin{cases} 0 & ( x \leq k ) \\ 12 ( x - k ) & ( x > k ) \end{cases} \end{aligned}$$ and satisfy the following conditions. (가) The function $f ( x )$ is differentiable on the entire set of real numbers. (나) For all real numbers $x$, $f ( x ) \geq g ( x )$. When the minimum value of $k$ is $\frac { q } { p }$, find the value of $a + p + q$. (Here, $p$ and $q$ are coprime natural numbers.) [4 points]
csat-suneung 2019 Q9 Determine parameters from given extremum conditions
For the function $f ( x ) = x ^ { 3 } - 3 x + a$, when the local maximum value is 7, what is the value of the constant $a$?
(1) 1
(2) 2
(3) 3
(4) 4
(5) 5
csat-suneung 2019 Q30 4 marks Geometric or applied optimisation problem
A cubic function $f ( x )$ with leading coefficient 1 and a quadratic function $g ( x )$ with leading coefficient $-1$ satisfy the following conditions. (가) The tangent line to the curve $y = f ( x )$ at the point $( 0,0 )$ and the tangent line to the curve $y = g ( x )$ at the point $( 2,0 )$ are both the $x$-axis. (나) The number of tangent lines to the curve $y = f ( x )$ drawn from the point $( 2,0 )$ is 2. (다) The equation $f ( x ) = g ( x )$ has exactly one real root. For all real numbers $x > 0$, $$g ( x ) \leq k x - 2 \leq f ( x )$$ Let $\alpha$ and $\beta$ be the maximum and minimum values of the real number $k$ satisfying the above inequality, respectively. When $\alpha - \beta = a + b \sqrt { 2 }$, find the value of $a ^ { 2 } + b ^ { 2 }$. (Here, $a$ and $b$ are rational numbers.) [4 points]
csat-suneung 2020 Q11 3 marks Find concavity, inflection points, or second derivative properties
How many integers $a$ are there such that the curve $y = a x ^ { 2 } - 2 \sin 2 x$ has an inflection point? [3 points]
(1) 4
(2) 5
(3) 6
(4) 7
(5) 8
csat-suneung 2020 Q12 3 marks Determine parameters from given extremum conditions
The function $f ( x ) = - x ^ { 4 } + 8 a ^ { 2 } x ^ { 2 } - 1$ has local maxima at $x = b$ and $x = 2 - 2 b$. What is the value of $a + b$? (Note: $a , b$ are constants with $a > 0 , b > 1$.) [3 points]
(1) 3
(2) 5
(3) 7
(4) 9
(5) 11
csat-suneung 2020 Q30 4 marks Determine parameters from given extremum conditions
A cubic function $f ( x )$ with positive leading coefficient satisfies the following conditions. (가) The equation $f ( x ) - x = 0$ has exactly 2 distinct real roots. (나) The equation $f ( x ) + x = 0$ has exactly 2 distinct real roots. When $f ( 0 ) = 0$ and $f ^ { \prime } ( 1 ) = 1$, find the value of $f ( 3 )$. [4 points]
csat-suneung 2021 Q20 4 marks Find critical points and classify extrema of a given function
For a real number $a$ ($a > 1$), define the function $f ( x )$ as $$f ( x ) = ( x + 1 ) ( x - 1 ) ( x - a)$$ Define the function $$g ( x ) = x ^ { 2 } \int _ { 0 } ^ { x } f ( t ) d t - \int _ { 0 } ^ { x } t ^ { 2 } f ( t ) d t$$ such that $g ( x )$ has exactly one extremum. What is the maximum value of $a$? [4 points]
(1) $\frac { 9 \sqrt { 2 } } { 8 }$
(2) $\frac { 3 \sqrt { 6 } } { 4 }$
(3) $\frac { 3 \sqrt { 2 } } { 2 }$
(4) $\sqrt { 6 }$
(5) $2 \sqrt { 2 }$
csat-suneung 2021 Q25 3 marks Count or characterize roots using extremum values
Find the positive value of $k$ such that the curve $y = 4 x ^ { 3 } - 12 x + 7$ and the line $y = k$ intersect at exactly 2 points. [3 points]
csat-suneung 2021 Q28 4 marks Determine parameters from given extremum conditions
For two constants $a$ and $b$ with $a < b$, define the function $f ( x )$ as $$f ( x ) = ( x - a ) ( x - b ) ^ { 2 }$$ For the inverse function $g ^ { - 1 } ( x )$ of the function $g ( x ) = x ^ { 3 } + x + 1$, the composite function $h ( x ) = \left( f \circ g ^ { - 1 } \right) ( x )$ satisfies the following conditions. Find the value of $f ( 8 )$. [4 points] (가) The function $( x - 1 ) | h ( x ) |$ is differentiable on the set of all real numbers. (나) $h ^ { \prime } ( 3 ) = 2$
csat-suneung 2021 Q30 4 marks Composite or piecewise function extremum analysis
The function $f ( x )$ is a cubic function with leading coefficient 1, and the function $g ( x )$ is a linear function. Define the function $h ( x )$ as $$h ( x ) = \begin{cases} | f ( x ) - g ( x ) | & ( x < 1 ) \\ f ( x ) + g ( x ) & ( x \geq 1 ) \end{cases}$$ If $h ( x )$ is differentiable on the entire set of real numbers, and $h ( 0 ) = 0$, $h ( 2 ) = 5$, find the value of $h ( 4 )$. [4 points]
csat-suneung 2022 Q6 3 marks Count or characterize roots using extremum values
How many integers $k$ are there such that the equation $2 x ^ { 3 } - 3 x ^ { 2 } - 12 x + k = 0$ has three distinct real roots? [3 points]
(1) 20
(2) 23
(3) 26
(4) 29
(5) 32
csat-suneung 2022 Q19 3 marks Determine intervals of increase/decrease or monotonicity conditions
Find the maximum value of the real number $a$ such that the function $f ( x ) = x ^ { 3 } + a x ^ { 2 } - \left( a ^ { 2 } - 8 a \right) x + 3$ is increasing on the entire set of real numbers. [3 points]
csat-suneung 2022 Q22 4 marks Determine parameters from given extremum conditions
For a cubic function $f ( x )$ with leading coefficient $\frac { 1 } { 2 }$ and a real number $t$, let $g ( t )$ be the number of real roots of the equation $f ^ { \prime } ( x ) = 0$ in the closed interval $[ t , t + 2 ]$. The function $g ( t )$ satisfies the following conditions.
(a) For all real numbers $a$, $\lim _ { t \rightarrow a + } g ( t ) + \lim _ { t \rightarrow a - } g ( t ) \leq 2$.
(b) $g ( f ( 1 ) ) = g ( f ( 4 ) ) = 2 , g ( f ( 0 ) ) = 1$ Find the value of $f ( 5 )$. [4 points]
csat-suneung 2023 Q6 3 marks Determine parameters from given extremum conditions
The function $f ( x ) = 2 x ^ { 3 } - 9 x ^ { 2 } + a x + 5$ has a local maximum at $x = 1$ and a local minimum at $x = b$. What is the value of $a + b$? (Here, $a$ and $b$ are constants.) [3 points]
(1) 12
(2) 14
(3) 16
(4) 18
(5) 20
csat-suneung 2024 Q7 3 marks Find critical points and classify extrema of a given function
For the function $f(x) = \frac{1}{3}x^3 - 2x^2 - 12x + 4$, if $f$ has a local maximum at $x = \alpha$ and a local minimum at $x = \beta$, find the value of $\beta - \alpha$. (Here, $\alpha$ and $\beta$ are constants.) [3 points]
(1) $-4$
(2) $-1$
(3) 2
(4) 5
(5) 8
csat-suneung 2024 Q21 4 marks Composite or piecewise function extremum analysis
For a positive number $a$, the function $f(x)$ defined on $x \geq -1$ is $$f(x) = \begin{cases} -x^2 + 6x & (-1 \leq x < 6) \\ a\log_4(x-5) & (x \geq 6) \end{cases}$$ For a real number $t \geq 0$, let $g(t)$ denote the maximum value of $f(x)$ on the closed interval $[t-1, t+1]$. If the minimum value of the function $g(t)$ on the interval $[0, \infty)$ is 5, find the minimum value of the positive number $a$. [4 points]
csat-suneung 2024 Q22 4 marks Determine parameters from given extremum conditions
A cubic function $f(x)$ with leading coefficient 1 satisfies the following condition.
For the function $f(x)$, $$f(k-1)f(k+1) < 0$$ has no integer solutions for $k$.
If $f'\left(-\frac{1}{4}\right) = -\frac{1}{4}$ and $f'\left(\frac{1}{4}\right) < 0$, find the value of $f(8)$. [4 points]
csat-suneung 2025 Q15 4 marks Determine parameters from given extremum conditions
For a constant $a$ ($a \neq 3\sqrt{5}$) and a quadratic function $f(x)$ with negative leading coefficient, the function $$g(x) = \begin{cases} x^{3} + ax^{2} + 15x + 7 & (x \leq 0) \\ f(x) & (x > 0) \end{cases}$$ satisfies the following conditions. (가) The function $g(x)$ is differentiable on the set of all real numbers. (나) The equation $g'(x) \times g'(x - 4) = 0$ has exactly 4 distinct real roots. What is the value of $g(-2) + g(2)$? [4 points]
(1) 30
(2) 32
(3) 34
(4) 36
(5) 38
csat-suneung 2025 Q19 3 marks Determine parameters from given extremum conditions
For a positive number $a$, let the function $f(x)$ be $$f(x) = 2x^{3} - 3ax^{2} - 12a^{2}x$$ When the local maximum value of $f(x)$ is $\frac{7}{27}$, what is the value of $f(3)$? [3 points]
csat-suneung 2026 Q19 3 marks Find absolute extrema on a closed interval or domain
For all real numbers $x$ with $- 2 \leq x \leq 2$, the inequality $$- k \leq 2 x ^ { 3 } + 3 x ^ { 2 } - 12 x - 8 \leq k$$ holds. Find the minimum value of the positive number $k$. [3 points]
gaokao 2015 Q9 Determine intervals of increase/decrease or monotonicity conditions
9. If the function $f ( x ) = \frac { 1 } { 2 } ( m - 2 ) x ^ { 2 } + ( n - 8 ) x + 1$ $(m \geq 0, n \geq 0)$ is monotonically decreasing on the interval $\left[ \frac { 1 } { 2 }, 2 \right]$, then the maximum value of $m n$ is
(A) $16$
(B) $18$
(C) $25$
(D) $\frac { 81 } { 2 }$
gaokao 2015 Q10 Prove an inequality using calculus-based optimisation
10. If a function $f ( x )$ defined on $\mathbb{R}$ satisfies $f ( 0 ) = - 1$, and its derivative $f ^ { \prime } ( x )$ satisfies $f ^ { \prime } ( x ) > k > 1$, then among the following conclusions, the one that must be wrong is
A. $f \left( \frac { 1 } { k } \right) < \frac { 1 } { k }$
B. $f \left( \frac { 1 } { k } \right) > \frac { 1 } { k - 1 }$
C. $f \left( \frac { 1 } { k - 1 } \right) < \frac { 1 } { k - 1 }$
D. $f \left( \frac { 1 } { k - 1 } \right) > \frac { k } { k - 1 }$
Section II (Non-Multiple Choice Questions, 100 points)
II. Fill-in-the-Blank Questions: This section contains 5 questions, each worth 4 points, for a total of 20 points. Write your answers in the corresponding positions on the answer sheet.
gaokao 2015 Q17 Find absolute extrema on a closed interval or domain
17. Let a be a real number. The maximum value of the function $f ( x ) = \left| x ^ { 2 } - a x \right|$ on the interval $[ 0,1 ]$ is denoted by $g ( a )$. When $a =$ $\_\_\_\_$,
$$\mathbf { y }$$
the value of $g ( a )$ is minimized. III. Solution Questions