# Thread: integrals pls help asap

1. ## integrals pls help asap

Estimate the area under the graph of f(x) = e^-x^2 from x = -2 to x = 2 using four approximating rectangles and midpoints.

2. Originally Posted by memena
Estimate the area under the graph of f(x) = e^-x^2 from x = -2 to x = 2 using four approximating rectangles and midpoints.

divide the interval [-2,2] into four equal sub-intervals. that is, we partition it into the intervals: [-2,-1], [-1,0], [0,1] and [1,2]

now, take the midpoints of these rectangles, and label them by $x_i$ for $i = 0,1,2,...$. so we have $x_0 = -1.5, ~x_1 = -0.5, ~x_2 = 0.5, \mbox{ and } x_3 = 1.5$

now we can use a finite Riemann sum to estimate the area. thus, an estimate of the area is given by:

$A = \sum_{i = 0}^3 f(x_i) \Delta x$

where $\Delta x = x_1 - x_0 = x_2 - x_1 = ... = 1$ and of course, $f(x) = e^{-x^2}$

3. to expound:

since $\Delta x = 1$, the area estimate is given by:

$\sum_{i = 0}^3 f(x_i) = f(x_0) + f(x_1) + f(x_2) + f(x_3)$

now i hope you can continue from there

if you are not clear on something, say so

4. ## ummm

ok do i draw this graph and then add the rectangles, if so can u tell me what it is supposed to look like, im sorry i am completely lost, i don't understand anything!

5. Originally Posted by memena
ok do i draw this graph Mr F says: Yes, use a graphics calculator.

and then add the rectangles, Mr F says: Yes, add the area of each of the four rectangles. As has already been stated, the total area is $f(x_0) + f(x_1) + f(x_2) + f(x_3)$, where $x_0 = -1.5, ~x_1 = -0.5, ~x_2 = 0.5 \,$ and $\, x_3 = 1.5$.

if so can u tell me what it is supposed to look like, Mr F says: Draw the graph (use a graphics calculator to get it) and draw in the rectangles. Then you'll know what it looks like.

im sorry i am completely lost, i don't understand anything! Mr F says: But you must surely have some examples to follow ....?
..

6. Originally Posted by memena
ok do i draw this graph and then add the rectangles, if so can u tell me what it is supposed to look like, im sorry i am completely lost, i don't understand anything!
yes, you can draw a diagram to visualize it.

here:

basically, your finding the area of the 4 rectangles we have in the diagram, that is, length times width for each. the height of each rectangle is given by $f(x_i)$, the widths are all one