this means, when A = 100, h = 10.
we are also told that . b is constant at 20cm, and so , but we wont even have to worry about that.
draw your triangle, and bisect the base with a vertical line going through the top vertice. is the base angle and we form a right triangle with the opposite side to theta being h and the adjacent side being 10.
Thus we have
now differentiate implicitly and continue