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A value of cc for which the minimum value of f(x)=x2−4cx+8cf(x) = x^2 - 4cx + 8c is greater than the maximum value of g(x)=−x2+3cx−2cg(x) = -x^2 + 3cx - 2c, is

Solution

✅ Correct Option: 3

We have two functions:

f(x)=x2−4cx+8cf(x) = x^2 - 4cx + 8c — a parabola opening upward, so it has a minimum.

g(x)=−x2+3cx−2cg(x) = -x^2 + 3cx - 2c — a parabola opening downward, so it has a maximum.

We need to find cc such that the minimum value of ff is greater than the maximum value of gg.


For an upward-opening quadratic ax2+bx+kax^2 + bx + k, the minimum value is k−b24ak - \dfrac{b^2}{4a}.

Here a=1, b=−4c, k=8ca = 1,\ b = -4c,\ k = 8c, so:

min⁡f=8c−(−4c)24(1)=8c−16c24=8c−4c2\min f = 8c - \dfrac{(-4c)^2}{4(1)} = 8c - \dfrac{16c^2}{4} = 8c - 4c^2

This comes from completing the square: f(x)=(x−2c)2+(8c−4c2)f(x) = (x - 2c)^2 + (8c - 4c^2). The squared part is always ≥0\geq 0, so the minimum value is 8c−4c28c - 4c^2.


For a downward-opening quadratic, the maximum value is k−b24ak - \dfrac{b^2}{4a}.

Here a=−1, b=3c, k=−2ca = -1,\ b = 3c,\ k = -2c, so:

max⁡g=−2c−(3c)24(−1)=−2c+9c24\max g = -2c - \dfrac{(3c)^2}{4(-1)} = -2c + \dfrac{9c^2}{4}

This comes from completing the square: g(x)=−(x−3c2)2+(−2c+9c24)g(x) = -\left(x - \dfrac{3c}{2}\right)^2 + \left(-2c + \dfrac{9c^2}{4}\right). The squared part is always ≤0\leq 0, so the maximum value is −2c+9c24-2c + \dfrac{9c^2}{4}.


Now we apply the condition min⁡f>max⁡g\min f > \max g:

8c−4c2>−2c+9c248c - 4c^2 > -2c + \dfrac{9c^2}{4}

Bringing everything to one side:

10c−4c2−9c24>010c - 4c^2 - \dfrac{9c^2}{4} > 0

Converting 4c2=16c244c^2 = \dfrac{16c^2}{4}:

10c−25c24>010c - \dfrac{25c^2}{4} > 0

Multiplying both sides by 44:

40c−25c2>040c - 25c^2 > 0

Factoring:

5c(8−5c)>05c(8 - 5c) > 0

A product of two factors is positive when both share the same sign:

c>0c > 0 and 8−5c>08 - 5c > 0 gives 0<c<850 < c < \dfrac{8}{5}

c<0c < 0 and 8−5c<08 - 5c < 0 gives c<0c < 0 and c>85c > \dfrac{8}{5}, which is impossible.

So the valid range is 0<c<850 < c < \dfrac{8}{5}.


Since 85=1.6\dfrac{8}{5} = 1.6, any value of cc strictly between 00 and 1.61.6 works.

Among the given options, c=12=0.5c = \dfrac{1}{2} = 0.5 lies in this interval.

c=12\boxed{c = \dfrac{1}{2}}

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