Mathematics · Oct / Nov 2020 · Paper 41 · Question 7
0580
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The question as text
Diagram 1 Diagram 2 Diagram 3 Diagram 4 These are the first four diagrams of a sequence. The diagrams are made from white dots and black dots.
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7(a)[2]
25 36 10 15 35 51 [2]
B1 for 3, 4 or 5 correct
7(b)[1]
n2 [1]
7(c)(i)[1]
92 [1]
7(c)(ii)[2]
1 2 (n2 – n) oe [2]
M1 for 1 2 (3n2 – n) – n2 oe or for final quadratic answer with 1 2 n2 oe 1 2 or − n oe but not both 2
7(d)[5]
a = 1 2 , b = 1 2 [5]
B2 for 2 correct equations eg a + b = 1, 8a + 4b = 6
or B1 for 1 correct equation
B2 for one correct value or M1 (dep on at least B1) for correctly eliminating one variable from two linear equations in a and b OR 1
B2 for a = 2
or B1 for 6a = 3 or for 3rd difference = 3 1
B2 for b = 2
or M1 for substituting their a into a correct equation of first differences
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