A-LevelMathematicsProbability and statistics9709/61 · Oct/Nov 2020 · Q45 Marks
The diagram shows the probability density function, f(x), of a random variable X. For 0 ≤ x ≤ a, f(x) = k; elsewhere f(x) = 0. [Figure X.X]
- (a)Express k in terms of a.[1]
- (b)Given that Var(X) = 3, find a.[4]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.
- [B1]
- [B1] FT OE seen. FT their k
- [M1] Attempt at correct integral and use of limits. Accept in terms of k or incorrect k.
- [M1] For subtracting mean², allow if integration not complete. FT incorrect values of k.
- [A1] Can be in terms of k.
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