A-LevelMathematicsProbability and statistics9709/62 · May/June 2017 · Q36 Marks
In a probability distribution the random variable X takes the value x with probability kx², where k is a constant and x takes values −2, −1, 2, 4 only.
- (i)Show that P(X = −2) has the same value as P(X = 2).[1]
- (ii)Draw up the probability distribution table for X, in terms of k, and find the value of k.[3]
- (iii)Find E(X).[2]
Mark scheme
Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.
- B1 need to see –2² k, 22k and 4k, algebraically correct expressions OE
- B1 -2, -1, 2, 4 only seen in a table, together with at least one attempted probability involving k
- M1 Summing 4 probs equating to 1. Must all be positive (table not required)
- A1 CWO
- M1 using Epx unsimplified. FT their k substituted before this stage, no inappropriate dividing
- A1
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