suppose a quantity y depends on another quantity x such that

 y is maximum at x1 ,and minimum at x2

at maximum or minimum the necessary condition is          dy / dx = 0

for maxima : dy/dx = 0                and d^2 y / dx^2 is negative

for minima:dy/dx=0                      and d^2 y /dx^2 is positive

at maximum or minimum ,the tangent to the curve at x1 or x2 is parallel to x-axis-------->slope =tan theeta =0                    as slope is dy/dx ------------->it is 0

jus before maximum slope is positive and at maximum zero and den negative ,hence secodn derivative at maximum becomes negative

jus before minimum ,slope is negaitve and at minimum zero and jus after minimum positive,hence secodn derivative there is positive

I HOPE EVERYTHING IS CLEAR-------->DRAW A GRAPH AND CHECK FOR THESE INTERPRETATIONS;THAT WOULD CLEAR IT OUT

DO RATE IF USEFUL