Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A train on a model railway leaves a station and speeds up, and a meter records its speed. Its speed is metres per second for the first , where is in seconds. A student wants the distance covered in these , and she cannot yet use an antiderivative, so she uses rectangles. She cuts the time into strips of width . The right-hand sum takes the speed at the end of each strip, and the left-hand sum takes the speed at the start. She knows that the exact distance is the area under the speed graph. She finds the right-hand sum, the left-hand sum, and then the exact distance with an antiderivative. Finally she notes that the exact distance lies between the two sums, because the speed is increasing. She would like to know that the exact value agrees with what the rectangles suggest. Find the right-hand sum, the left-hand sum, and the exact distance, each in metres.
Cut into strips of width . The right-hand sum is , and its limit is the integral.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. , strips of width , .
Plan. Add the right-hand and left-hand rectangles, then use the antiderivative.
Step 1. Right-hand sum. .
Step 2. Left-hand sum. .
Step 3. Exact distance. .
Answer: , and .
Check. , as expected for an increasing speed. The average of the two sums is , close to the exact distance.
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