find dy dx of the least value of a curve?

Answer 1

Under the right assumptions, it has to be equal to 0.

Part of learning calculus is being clear as to what those assumptions are.

Answer 2

If it's a local minimum over a differentiable function, the derivative of the function at that point is zero. (The tangent to the curve is horizontal.)

Answer 3

do
dy/dx=0
it will give you an equation,find its root and draw that on number line.....and -ve are the least point