maths problem

[elsen_678]

the graph of y=f(x) is concaved down over the interval (A,B)
ie f''(x)<0 for A=<x=<B
the tangent of the curve at point P(k,f(k)) meets lines x=A and x=B at P and Q respectively.
the value of k is such that the area bounded by the curve, the tangent and lines x=A and x=B is minimized.
Prove that k is independent of f(x).

somebody helps thanks

[da_willem]

Do you have no clue at all?

Begin with a drawing of the situation. Try to be more specific in your question. We're not going to do the problem for you...

[elsen_678]

i have done some sketche