We have two functions:
f(x)=x2−4cx+8c — a parabola opening upward, so it has a minimum.
g(x)=−x2+3cx−2c — a parabola opening downward, so it has a maximum.
We need to find c such that the minimum value of f is greater than the maximum value of g.
For an upward-opening quadratic ax2+bx+k, the minimum value is k−4ab2.
Here a=1, b=−4c, k=8c, so:
minf=8c−4(1)(−4c)2=8c−416c2=8c−4c2
This comes from completing the square: f(x)=(x−2c)2+(8c−4c2). The squared part is always ≥0, so the minimum value is 8c−4c2.
For a downward-opening quadratic, the maximum value is k−4ab2.
Here a=−1, b=3c, k=−2c, so:
maxg=−2c−4(−1)(3c)2=−2c+49c2
This comes from completing the square: g(x)=−(x−23c)2+(−2c+49c2). The squared part is always ≤0, so the maximum value is −2c+49c2.
Now we apply the condition minf>maxg:
8c−4c2>−2c+49c2
Bringing everything to one side:
10c−4c2−49c2>0
Converting 4c2=416c2:
10c−425c2>0
Multiplying both sides by 4:
40c−25c2>0
Factoring:
5c(8−5c)>0
A product of two factors is positive when both share the same sign:
c>0 and 8−5c>0 gives 0<c<58
c<0 and 8−5c<0 gives c<0 and c>58, which is impossible.
So the valid range is 0<c<58.
Since 58=1.6, any value of c strictly between 0 and 1.6 works.
Among the given options, c=21=0.5 lies in this interval.
c=21