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Let f(x)=f(x) = min{2x2,52−5x2x², 52-5x}, where x is any positive real number. Then the maximum possible value of f(x)f(x) is

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Solution

✅ Correct Answer: 32

We have f(x)=min⁡{2x2,52−5x}f(x) = \min\{2x^2, 52-5x\} where xx is any positive real number.

The function f(x)f(x) takes the smaller value between 2x22x^2 and 52−5x52-5x for any given xx.

2x22x^2 is a parabola opening upward (gets larger as xx increases)

52−5x52-5x is a decreasing straight line (gets smaller as xx increases)

Solution figure for CAT 2018 QA question 18 (Algebra)

The maximum value of f(x)f(x) occurs at the intersection point of these two functions.

Before the intersection: 2x2<52−5x2x^2 < 52-5x, so f(x)=2x2f(x) = 2x^2 (increasing)

After the intersection: 2x2>52−5x2x^2 > 52-5x, so f(x)=52−5xf(x) = 52-5x (decreasing)

At the intersection: Both functions are equal, giving us the peak of the minimum function. This is true for any min, max function.


We set the two functions equal:

2x2=52−5x2x^2 = 52-5x

2x2+5x−52=02x^2 + 5x - 52 = 0

2x2+13x−8x−52=02x^2 + 13x - 8x - 52 = 0

x(2x+13)−4(2x+13)=0x(2x + 13) - 4(2x + 13) = 0

(2x+13)(x−4)=0(2x + 13)(x - 4) = 0


From (2x+13)(x−4)=0(2x + 13)(x - 4) = 0:

2x+13=0→x=−1322x + 13 = 0 \rightarrow x = -\dfrac{13}{2}

x−4=0→x=4x - 4 = 0 \rightarrow x = 4

Since xx must be positive, we take x=4x = 4.


At x=4x = 4:

f(4)=2(4)2=2×16=32f(4) = 2(4)^2 = 2 \times 16 = 32

Therefore, the maximum possible value of f(x)f(x) is 3232.

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