CAIE
FP2
2011
November
Q7
7 The lifetime, in hours, of a 'Trulite' light bulb is a random variable \(T\). The probability density function f of \(T\) is given by
$$\mathrm { f } ( t ) = \begin{cases} 0 & t < 0
\lambda \mathrm { e } ^ { - \lambda t } & t \geqslant 0 \end{cases}$$
where \(\lambda\) is a positive constant. Given that the mean lifetime of Trulite bulbs is 2000 hours, find the probability that a randomly chosen Trulite bulb has a lifetime of at least 1000 hours.
A particular light fitting has 6 randomly chosen Trulite bulbs. Find the probability that no more than one of these bulbs has a lifetime less than 1000 hours.
By using new technology, the proportion of Trulite bulbs with very short lifetimes is to be reduced. Find the least value of the new mean lifetime that will ensure that the probability that a randomly chosen Trulite bulb has a lifetime of no more than 4 hours is less than 0.001 .
CAIE
FP2
2011
November
Q9
9 A random sample of five metal rods produced by a machine is taken. Each rod is tested for hardness. The results, in suitable units, are as follows.
$$\begin{array} { l l l l l }
524 & 526 & 520 & 523 & 530
\end{array}$$
Assuming a normal distribution, calculate a \(95 \%\) confidence interval for the population mean.
Some adjustments are made to the machine. Assume that a normal distribution is still appropriate and that the population variance remains unchanged. A second random sample, this time of ten metal rods, is now taken. The results for hardness are as follows.
$$\begin{array} { l l l l l l l l l l }
525 & 520 & 522 & 524 & 518 & 520 & 519 & 525 & 527 & 516
\end{array}$$
Stating suitable hypotheses, test at the \(10 \%\) significance level whether there is any difference between the population means before and after the adjustments.
CAIE
FP2
2012
November
Q1
1
\includegraphics[max width=\textwidth, alt={}, center]{34024618-0ff9-44a1-ac57-d4d7e8a3655e-2_216_1205_253_470}
A rigid body consists of two uniform circular discs, each of mass \(m\) and radius \(a\), the centres of which are rigidly attached to the ends \(A\) and \(B\) of a uniform rod of mass \(3 m\) and length \(10 a\). The discs and the rod are in the same plane and \(O\) is the point on the rod such that \(A O = 4 a\) (see diagram). Show that the moment of inertia of the body about an axis through \(O\) perpendicular to the plane of the discs is \(81 m a ^ { 2 }\).
CAIE
FP2
2012
November
Q3
3 A particle \(P\) of mass \(m\) is attached to one end of a light inextensible string of length \(a\). The other end of the string is attached to a fixed point \(O\). The particle is held with the string taut and horizontal and is then released. When the string is vertical, it comes into contact with a small smooth peg \(A\) which is vertically below \(O\) and at a distance \(x ( < a )\) from \(O\). In the subsequent motion, when \(A P\) makes an angle \(\theta\) with the downward vertical, the tension in the string is \(T\). Show that
$$T = m g \left( 3 \cos \theta + \frac { 2 x } { a - x } \right)$$
Given that \(P\) completes a vertical circle about \(A\), find the least possible value of \(\frac { x } { a }\).
CAIE
FP2
2012
November
Q8
8 The yield of a particular crop on a farm is thought to depend principally on the amount of sunshine during the growing season. For a random sample of 8 years, the average yield, \(y\) kilograms per square metre, and the average amount of sunshine per day, \(x\) hours, are recorded. The results are given in the following table.
| \(x\) | 12.2 | 10.4 | 5.2 | 6.3 | 11.8 | 10.0 | 14.2 | 2.3 |
| \(y\) | 15 | 9 | 10 | 7 | 8 | 11 | 12 | 6 |
$$\left[ \Sigma x = 72.4 , \Sigma x ^ { 2 } = 769.9 , \Sigma y = 78 , \Sigma y ^ { 2 } = 820 , \Sigma x y = 761.3 . \right]$$
- Find the equation of the regression line of \(y\) on \(x\).
- Find the product moment correlation coefficient.
- Test, at the \(5 \%\) significance level, whether there is positive correlation between the average yield and the average amount of sunshine per day.
CAIE
FP2
2012
November
Q9
9 marks
9 The leaves from oak trees growing in two different areas \(A\) and \(B\) are being measured. The lengths, in cm , of a random sample of 7 oak leaves from area \(A\) are
$$6.2 , \quad 8.3 , \quad 7.8 , \quad 9.3 , \quad 10.2 , \quad 8.4 , \quad 7.2$$
Assuming that the distribution is normal, find a 95\% confidence interval for the mean length of oak leaves from area \(A\).
The lengths, in cm, of a random sample of 5 oak leaves from area \(B\) are
$$5.9 , \quad 7.4 , \quad 6.8 , \quad 8.2 , \quad 8.7$$
Making suitable assumptions, which should be stated, test, at the \(5 \%\) significance level, whether the mean length of oak leaves from area \(A\) is greater than the mean length of oak leaves from area \(B\). [9]
CAIE
FP2
2012
November
Q10 OR
A continuous random variable \(X\) is believed to have the probability density function f given by
$$f ( x ) = \begin{cases} \frac { 3 } { 10 } \left( 5 x - x ^ { 2 } - 4 \right) & 2 \leqslant x < 4
0 & \text { otherwise } \end{cases}$$
A random sample of 60 observations was taken and these values are summarised in the following grouped frequency table.
| Interval | \(2 \leqslant x < 2.4\) | \(2.4 \leqslant x < 2.8\) | \(2.8 \leqslant x < 3.2\) | \(3.2 \leqslant x < 3.6\) | \(3.6 \leqslant x < 4\) |
| Observed frequency | 19 | 17 | 16 | 8 | 0 |
The estimated mean, based on the grouped data in the table above, is 2.69 , correct to 2 decimal places. It is decided that a goodness of fit test will only be conducted if the mean predicted from the probability density function is within \(10 \%\) of the estimated mean. Show that this condition is satisfied.
The relevant expected frequencies are as follows.
| Interval | \(2 \leqslant x < 2.4\) | \(2.4 \leqslant x < 2.8\) | \(2.8 \leqslant x < 3.2\) | \(3.2 \leqslant x < 3.6\) | \(3.6 \leqslant x < 4\) |
| Expected frequency | 15.456 | 16.032 | 14.304 | 10.272 | 3.936 |
Show how the expected frequency for the interval \(3.2 \leqslant x < 3.6\) is obtained.
Carry out the goodness of fit test at the 10\% significance level.
CAIE
FP2
2012
November
Q1
1
\includegraphics[max width=\textwidth, alt={}, center]{d3e9a568-a9ea-483e-8e65-90fdc4a69781-2_216_1205_253_470}
A rigid body consists of two uniform circular discs, each of mass \(m\) and radius \(a\), the centres of which are rigidly attached to the ends \(A\) and \(B\) of a uniform rod of mass \(3 m\) and length \(10 a\). The discs and the rod are in the same plane and \(O\) is the point on the rod such that \(A O = 4 a\) (see diagram). Show that the moment of inertia of the body about an axis through \(O\) perpendicular to the plane of the discs is \(81 m a ^ { 2 }\).