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The cross-sectional view of a tunnel is shown on the axes below. The line [AB] representsa vertical wall located at the left side of the tunnel. The height, in metres, of the tunnel abovethe horizontal ground is modelled by y = -0.1 x 3 + 0.8 x 2 , 2 ≤ x ≤ 8 , relative to an origin O.yx2 4 6 8246A108EBCDOPoint A has coordinates (2 , 0), point B has coordinates (2 , 2.4), and point C hascoordinates (8 , 0).(a) (i) Find dy___dx .(ii) Hence find the maximum height of the tunnel. [6](b) Find the height of the tunnel when(i) x = 4 .(ii) x = 6 . [3](c) Use the trapezoidal rule, with three intervals, to estimate the cross-sectional area ofthe tunnel. [3](d) (i) Write down the integral which can be used to find the cross-sectional area ofthe tunnel.(ii) Hence find the cross-sectional area of the tunnel. [4]

Question

The cross-sectional view of a tunnel is shown on the axes below. The line [AB] representsa vertical wall located at the left side of the tunnel. The height, in metres, of the tunnel abovethe horizontal ground is modelled by y = -0.1 x 3 + 0.8 x 2 , 2 ≤ x ≤ 8 , relative to an origin O.yx2 4 6 8246A108EBCDOPoint A has coordinates (2 , 0), point B has coordinates (2 , 2.4), and point C hascoordinates (8 , 0).(a) (i) Find dy___dx .(ii) Hence find the maximum height of the tunnel. 6 Find the height of the tunnel when(i) x = 4 .(ii) x = 6 . 3 Use the trapezoidal rule, with three intervals, to estimate the cross-sectional area ofthe tunnel. 3 (i) Write down the integral which can be used to find the cross-sectional area ofthe tunnel.(ii) Hence find the cross-sectional area of the tunnel. [4]

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Solution

(a) (i) The derivative of y with respect to x, dy/dx, is found by differentiating the equation y = -0.1x^3 + 0.8x^2. Using the power rule for differentiation, we get dy/dx = -0.3x^2 + 1.6x.

(ii) To find the maximum height of the tunnel, we need to find the x-value at which dy/dx = 0 (since the slope of the tangent line is zero at a maximum). Setting -0.3x^2 + 1.6x = 0 and solving for x gives x = 0 or x = 1.6/0.3 = 5.33. Substituting x = 5.33 into the equation for y gives the maximum height of the tunnel.

(b) To find the height of the tunnel when x = 4 and x = 6, substitute these values into the equation for y.

(c) The trapezoidal rule is a method for approximating the definite integral of a function. It works by dividing the area under the curve into trapezoids and then adding up the areas of these trapezoids. With three intervals, the x-values are 2, 4, 6, and 8. The corresponding y-values are found by substituting these x-values into the equation for y. The area is then approximated as (b-a)/2n [f(x0) + 2f(x1) + 2f(x2) + ... + 2f(xn-1) + f(xn)], where a and b are the limits of integration, n is the number of intervals, and f(x) is the function.

(d) (i) The integral which can be used to find the cross-sectional area of the tunnel is the definite integral of the function y = -0.1x^3 + 0.8x^2 from x = 2 to x = 8.

(ii) To find the cross-sectional area of the tunnel, evaluate this definite integral.

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