Summation or sigma notation is a way to express a long sum into a single compact expression.FINDING THE AREA UNDER THE CURVE OF A FUNCTION BY USING RECTANGLES Given a continuous function that is defined over an interval [a,b], we can approximate or find the exact the area under the curve of f(x) by using Riemann sums. This can be done by evaluating a lower or upper Sum of areas of (rectangles that lie above or below the graph) of f(x).Finally, find the exact area by evaluating: STEPS: The proces