Need help estimating an area under a curve

[viciousp]

Homework Statement

The graph is the graph of the rate of increase or decrease of the river throughout a 9 day period. At day 0 the height is at 111 feet. What day is the river crest at 114.4 feet.

http://img135.imageshack.us/img135/8987/mathgraphoo9.jpg
http://g.imageshack.us/img135/mathgraphoo9.jpg/1/

Homework Equations

3. Attempt at Solution

I know you have to estimate the area under the curve, I used rectangles and got a estimate between the 4th and 5th day. I am just asking if anyone got something similar to that or if I am way off. Thank you in advance

 

[mgb_phys]

If you have the raw data there are a few numerical techniques that can do better ( see Newton-Raphson) but if you just have that chart then the best you can do is to count squares/triangles on paper - for more accuracy you just need smaller squares.