Given: 0; -1; 1; 6; 14; ...
2.1.1 Show that this sequence has a constant second difference.
2.1.2 Write down the next term of the sequence.
2.1.3 Determine an exp... show full transcript
Worked Solution & Example Answer:Given: 0; -1; 1; 6; 14; .. - NSC Mathematics - Question 2 - 2016 - Paper 1
Step 1
Show that this sequence has a constant second difference.
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Answer
To show that the sequence has a constant second difference, we first calculate the first and second differences:
First Differences:
( -1 - 0 = -1 )
( 1 - (-1) = 2 )
( 6 - 1 = 5 )
( 14 - 6 = 8 )
First differences: ( -1, 2, 5, 8 )
Second Differences:
( 2 - (-1) = 3 )
( 5 - 2 = 3 )
( 8 - 5 = 3 )
Second differences: ( 3, 3, 3 )
Since the second differences are constant (equal to 3), the sequence has a constant second difference.
Step 2
Write down the next term of the sequence.
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Answer
From the first differences, we see a pattern.
The last first difference is 8, so the next first difference will be ( 8 + 3 = 11 ).
Thus, the next term can be calculated as follows:
[ 14 + 11 = 25 ]
Therefore, the next term of the sequence is 25.
Step 3
Determine an expression for the nth term of the sequence.
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Answer
Given that the second difference is constant, we can express the nth term as:
[ T_n = a n^2 + b n + c ]
We can calculate coefficients a, b, and c using the known terms:
For n = 1: ( T_1 = 0 ) => ( a + b + c = 0 )
For n = 2: ( T_2 = -1 ) => ( 4a + 2b + c = -1 )
For n = 3: ( T_3 = 1 ) => ( 9a + 3b + c = 1 )
Solving these equations, we find:
[ a = \frac{3}{2}, b = -\frac{11}{2}, c = 4 ]
Thus, the nth term is:
[ T_n = \frac{3}{2}n^2 - \frac{11}{2}n + 4 ]
Step 4
Calculate the 30th term.
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Answer
Using the expression for the nth term:
[ T_{30} = \frac{3}{2}(30^2) - \frac{11}{2}(30) + 4 ]
Calculating further:
[ T_{30} = \frac{3}{2}(900) - \frac{11}{2}(30) + 4 ]
[ = 1350 - 165 + 4 ]
[ = 1189 ]
Thus, the 30th term is 1189.
Step 5
Prove that a = 6 and b = 20.
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Answer
From the series:
[ a + 13 + b + 27 + ... ]
Using the information given:
The sum of first two terms is:
[ a + 13 = 6 + 13 \Rightarrow a = 6 ]
For b, we see:
[ b + 27 = 20 + 27 \Rightarrow b = 20 ]
Thus, it is proven that a = 6 and b = 20.
Step 6
Determine which term of the series will be equal to 230.
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Answer
To find the term equal to 230:
[ S_n = a + (n-1)d ] where ( a = 6, d = b + 13 - 6 = 20 - 6 = 14 )
Set this equal to 230:
[ 6 + (n-1) * 14 = 230 ]
[ (n-1) * 14 = 224 ]
[ n-1 = 16 \Rightarrow n = 17 ]
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Answer
The series converges if the terms approach zero as ( n \to \infty ).
We can use the convergence test:
[ 1 - \frac{k}{5}, ; \left(1 - \frac{k^2}{5}\right), ; \left(1 - \frac{k^3}{5}\right) ] will converge if:
[ |1 - \frac{k}{5}| < 1 \Rightarrow |k| < 10 ]
Thus, the series converges for ( k < 10 ).
Step 8
Determine the sum of the first 40 terms of the series, to the nearest integer.
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Answer
Using the formula for the sum of the first n terms:
[ S_n = \frac{n}{2} (a + l) ] where ( a = 16 ) and ( l = a + (n-1)d ) for the arithmetic series.
Calculating:
Find d:
[ d = 3 - 16 = -13 ]
Find l for n = 40:
[ l = 16 + (40-1)(-13) = 16 - 507 = -491 ]
Thus the sum S_40 is:
[ S_{40} = \frac{40}{2} (16 - 491) = 20 * (-475) = -9500 ]
Therefore, the sum of the first 40 terms is -9500.
Step 9
Write the series: 16 + 8 + 4 + 2 + ... in the form \( \sum_{k} T_k \).
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Answer
The series can be written as:
[ T_k = 16 \cdot (\frac{1}{2})^{k-1} ]
Thus:
[ S = \sum_{k=1}^{\infty} T_k = \sum_{k=1}^{\infty} 16 \cdot (\frac{1}{2})^{k-1} ]
Step 10
Determine S_0 of the series in QUESTION 2.4.2.
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To determine ( S_0 ):
This series converges to:
[ S_0 = \frac{a}{1 - r} ; ext{where} , a = 16, r = \frac{1}{2} ]
Therefore:
[ S_0 = \frac{16}{1 - \frac{1}{2}} = \frac{16}{\frac{1}{2}} = 32 ]