A is the area, h is the height, b is the length of the base and is the base angle.

recall that

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