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Trapezium Rule C2 AQA

This should be simple but I can't get to the right answer.

Q) Using the trapezium rule with 3 ordinates, find an estimate for the area under the curve y = x^2 - 3x from x = 4 to x = 8.

If there are 3 ordinates, you do 4 (8-4) divided by 3 right? I think that's where I'm going wrong. Then input 4+1.333 and the answer of that + 1.333 as my middle ordinates then x=4 and x=8 as my end points, so I get:

4 + 2(12.44 + 24.44) + 40 giving me 117... the answer is apparently 80.
Reply 1
Avatar for mr tim
mr tim
18
you need to do with x = 4,6,8 which would mean that h=2

then you use the trapezium rule with those values.
Reply 2
Avatar for JoeUtd
JoeUtd
OP
Doing so, I still only get 24. Where am I going wrong?
Reply 3
Avatar for mr tim
mr tim
18
did you use this formulae:

h2(y0+yn+2(y1))\frac{h}{2}(y_0+y_n+2(y_1))

where h = difference in the x values = 2
y0 = y value at x = 4
y1 = y value at x = 6
yn = y value at x = 8

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