Use the trapezium rule with four intervals to find an approximation of $$ \int^{2}_{-2} \frac{1}{x^2+1} \; \text{dx} $$ |
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Sometimes the area under a curve cannot be found by integration. In these cases a method to approximate the area under the curve called the trapezium rule can be used.
The rule divides the area under a curve into trapeziums, calculates their areas, then sums these areas to get an approsimation of the total area.
Area of a trapezium
Trapezium Rule
If the width of each interval is \(h\) and the y values (ordinates) are denoted as \(y_0, y_1, y_2, y_3 ...\) then the formula for finding the sum of the areas of these trapeziums is
$$ \frac12 h ((y_0 + y_n) + 2(y_1 + y_2 + ... + y_{n-1})) $$
where n is the number of intervals.
Note that the number or ordinates is always one more than the number of intervals.
If the lower bound of the required area is \(p\) and the upper bound is \(q\) then
$$ h= \frac{q-p}{n} $$
The more trapeziums the area is divided into the more accurate the estimate.
When the gradient of the graph is increasing over the given interval the area given by the trapezium rule will be an overestimate of the actual area.
When the gradient of the graph is decreasing over the given interval the area given by the trapezium rule will be an underestimate of the actual area.
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