The total area under y=f(x) on an interval is approximated by
n∑i=1f(x∗i)Δx,
which is the sum of the areas of n rectangles.
This sort of expression is called a Riemann Sum.
We use the Greek letter sigma (Σ) to mean "sum". The expression
"n∑i=1(formula involving i)" means
"plug i=1 into the formula, then plug in i=2, all the way up to i=n,
and add up the terms."
The notation for Riemann Sums can be complicated.
Keep referring to the left side for a list of symbols.
Notation:
a is the starting point; b is the end point.
n is the number of pieces in which the interval [a,b] is subdivided.
Δx=b−an is the size of each of those sub-intervals.
[xi−1,xi] is the ith interval; in particular
x0=a, x1=a+Δx, …, xi=a+iΔx, xn=b.
x∗i is a representative from the ith interval
f(x∗i) is the height of the rectangle Ri over the ith interval.
f(x∗i)Δx is the area of Ri.
The exact area is the limit of the Riemann sum as n→∞.
Notice that we could use the left endpointxi−1, the right endpointxi, the midpointxi−1+xi2, or any other representative point.
While each choice will give us different approximations, they will all give us the same answer at the limit!