IB DP Math AI: Topic : SL 1.5: Laws of exponents: IB style Questions HL Paper 2

Question 4. [Maximum mark: 18]

In a small village there are two doctors’ clinics, one owned by Doctor Black and the other owned by Doctor Green. It was noted after each year that 3.5 % of Doctor Black’s patients moved to Doctor Green’s clinic and 5 % of Doctor Green’s patients moved to Doctor Black’s clinic. All additional losses and gains of patients by the clinics may be ignored.

At the start of a particular year, it was noted that Doctor Black had 2100 patients on their register, compared to Doctor Green’s 3500 patients.

a. Write down a transition matrix T indicating the annual population movement between clinics. [2]

b.  Find a prediction for the ratio of the number of patients Doctor Black will have, compared to Doctor Green, after two years. [2]

c.  Find a matrix P , with integer elements, such that T = PDP1 , where D is a diagonal matrix.

d.  Hence, show that the long-term transition matrix T x is given by T x =

e.  Hence, or otherwise, determine the expected ratio of the number of patients Doctor Black would have compared to Doctor Green in the long term. [2]

▶️Answer/Explanation

(a)

so ratio is  \(2294 : 3306 = (1147 :1653, 0.693889…)\) 

(c) to solve Ax =  λx : \( (0.965 – λ) (0.95 – λ) -0.05 × 0.035 = 0\)

\(λ = 0.915 OR λ =1\)

attempt to find eigenvectors for at least one eigenvalue when \(λ = 0.915\) , \(x=(_{-1}^{1})\)

(or any real multiple) when λ =1, \(x=(_{7}^{10})\) therefore (accept integer valued multiples of their eigenvectors and columns in either order)

(e) so ratio is \(3294 : 2306\)   \((1647 :1153, 1.42844…, 0.700060…)\)

Question

The following table shows values of ln x and ln y.

The relationship between ln x and ln y can be modelled by the regression equation ln y = a ln x + b.

a.Find the value of a and of b.[3]

b.Use the regression equation to estimate the value of y when x = 3.57.[3]

c.The relationship between x and y can be modelled using the formula y = kxn, where k ≠ 0 , n ≠ 0 , n ≠ 1.

By expressing ln y in terms of ln x, find the value of n and of k.[7]

 
▶️Answer/Explanation

Markscheme

valid approach (M1)

eg one correct value

−0.453620, 6.14210

a = −0.454, b = 6.14 A1A1 N3

[3 marks]

a.

correct substitution (A1)

eg −0.454 ln 3.57 + 6.14

correct working (A1)

eg ln y = 5.56484

261.083 (260.409 from 3 sf)

y = 261, (y = 260 from 3sf) A1 N3

Note: If no working shown, award N1 for 5.56484.
If no working shown, award N2 for ln y = 5.56484.

[3 marks]

b.

METHOD 1

valid approach for expressing ln y in terms of ln x (M1)

eg \({\text{ln}}\,y = {\text{ln}}\,\left( {k{x^n}} \right),\,\,{\text{ln}}\,\left( {k{x^n}} \right) = a\,{\text{ln}}\,x + b\)

correct application of addition rule for logs (A1)

eg \({\text{ln}}\,k + {\text{ln}}\,\left( {{x^n}} \right)\)

correct application of exponent rule for logs A1

eg \({\text{ln}}\,k + n\,{\text{ln}}\,x\)

comparing one term with regression equation (check FT) (M1)

eg \(n = a,\,\,b = {\text{ln}}\,k\)

correct working for k (A1)

eg \({\text{ln}}\,k = 6.14210,\,\,\,k = {e^{6.14210}}\)

465.030

\(n = – 0.454,\,\,k = 465\) (464 from 3sf) A1A1 N2N2

METHOD 2

valid approach (M1)

eg \({e^{{\text{ln}}\,y}} = {e^{a\,{\text{ln}}\,x + b}}\)

correct use of exponent laws for \({e^{a\,{\text{ln}}\,x + b}}\) (A1)

eg \({e^{a\,{\text{ln}}\,x}} \times {e^b}\)

correct application of exponent rule for \(a\,{\text{ln}}\,x\) (A1)

eg \({\text{ln}}\,{x^a}\)

correct equation in y A1

eg \(y = {x^a} \times {e^b}\)

comparing one term with equation of model (check FT) (M1)

eg \(k = {e^b},\,\,n = a\)

465.030

\(n = – 0.454,\,\,k = 465\) (464 from 3sf) A1A1 N2N2

METHOD 3

valid approach for expressing ln y in terms of ln x (seen anywhere) (M1)

eg \({\text{ln}}\,y = {\text{ln}}\,\left( {k{x^n}} \right),\,\,{\text{ln}}\,\left( {k{x^n}} \right) = a\,{\text{ln}}\,x + b\)

correct application of exponent rule for logs (seen anywhere) (A1)

eg \({\text{ln}}\,\left( {{x^a}} \right) + b\)

correct working for b (seen anywhere) (A1)

eg \(b = {\text{ln}}\,\left( {{e^b}} \right)\)

correct application of addition rule for logs A1

eg \({\text{ln}}\,\left( {{e^b}{x^a}} \right)\)

comparing one term with equation of model (check FT) (M1)

eg \(k = {e^b},\,\,n = a\)

465.030

\(n = – 0.454,\,\,k = 465\) (464 from 3sf) A1A1 N2N2

[7 marks]

c.

Question

a.Expand \({(x – 2)^4}\) and simplify your result.[3]

b.Find the term in \({x^3}\) in \((3x + 4){(x – 2)^4}\) .[3]

▶️Answer/Explanation

Markscheme

evidence of expanding     M1

e.g. \({(x – 2)^4} = {x^4} + 4{x^3}( – 2) + 6{x^2}{( – 2)^2} + 4x{( – 2)^3} + {( – 2)^4}\)     A2     N2

\({(x – 2)^4} = {x^4} – 8{x^3} + 24{x^2} – 32x + 16\)

[3 marks]

a.

finding coefficients, \(3 \times 24( = 72)\) , \(4 \times( – 8)( = – 32)\)     (A1)(A1)

term is \(40{x^3}\)     A1     N3

[3 marks]

b.

Question

Let \(f(x) = {\log _3}\frac{x}{2} + {\log _3}16 – {\log _3}4\) , for \(x > 0\) .

a.Show that \(f(x) = {\log _3}2x\) .[2]

b.Find the value of \(f(0.5)\) and of \(f(4.5)\) .[3]

c(i), (ii) and (iii).The function f can also be written in the form \(f(x) = \frac{{\ln ax}}{{\ln b}}\) .

(i)     Write down the value of a and of b .

(ii)    Hence on graph paper, sketch the graph of f , for \( – 5 \le x \le 5\) , \( – 5 \le y \le 5\) , using a scale of 1 cm to 1 unit on each axis.

(iii)   Write down the equation of the asymptote.[6]

d.Write down the value of \({f^{ – 1}}(0)\) .[1]

e.The point A lies on the graph of f . At A, \(x = 4.5\) .

On your diagram, sketch the graph of \({f^{ – 1}}\) , noting clearly the image of point A.[4]

 
▶️Answer/Explanation

Markscheme

combining 2 terms     (A1)

e.g. \({\log _3}8x – {\log _3}4\) , \({\log _3}\frac{1}{2}x + {\log _3}4\)

expression which clearly leads to answer given     A1

e.g. \({\log _3}\frac{{8x}}{4}\) , \({\log _3}\frac{{4x}}{2}\)

\(f(x) = {\log _3}2x\)     AG     N0

[2 marks]

a.

attempt to substitute either value into f     (M1)

e.g. \({\log _3}1\) , \({\log _3}9\)

\(f(0.5) = 0\) , \(f(4.5) = 2\)     A1A1     N3

[3 marks]

b.

(i) \(a = 2\) , \(b = 3\)     A1A1     N1N1

(ii)
     A1A1A1     N3

Note: Award A1 for sketch approximately through \((0.5 \pm 0.1{\text{, }}0 \pm 0.1)\) , A1 for approximately correct shape, A1 for sketch asymptotic to the y-axis.

(iii) \(x = 0\) (must be an equation)     A1     N1

[6 marks]

c(i), (ii) and (iii).

\({f^{ – 1}}(0) = 0.5\)     A1     N1

[1 mark]

d.


     A1A1A1A1     N4

Note: Award A1 for sketch approximately through \((0 \pm 0.1{\text{, }}0.5 \pm 0.1)\) , A1 for approximately correct shape of the graph reflected over \(y = x\) , A1 for sketch asymptotic to x-axis, A1 for point \((2 \pm 0.1{\text{, }}4.5 \pm 0.1)\) clearly marked and on curve.

[4 marks]

e.

Question

The mass \(M\) of a decaying substance is measured at one minute intervals. The points \((t,{\text{ }}\ln M)\) are plotted for \(0 \leqslant t \leqslant 10\), where \(t\) is in minutes. The line of best fit is drawn. This is shown in the following diagram.

M16/5/MATME/SP2/ENG/TZ1/05

The correlation coefficient for this linear model is \(r =  – 0.998\).

a.State two words that describe the linear correlation between \(\ln M\) and \(t\).[2]

b.The equation of the line of best fit is \(\ln M =  – 0.12t + 4.67\). Given that \(M = a \times {b^t}\), find the value of \(b\).[4]

▶️Answer/Explanation

Markscheme

strong, negative (both required)     A2     N2

[2 marks]

a.

METHOD 1

valid approach     (M1)

eg\(\,\,\,\,\,\)\({{\text{e}}^{\ln M}} = {{\text{e}}^{ – 0.12t + 4.67}}\)

correct use of exponent laws for \({{\text{e}}^{ – 0.12t + 4.67}}\)     (A1)

eg\(\,\,\,\,\,\)\({{\text{e}}^{ – 0.12t}} \times {{\text{e}}^{4.67}}\)

comparing coefficients/terms     (A1)

eg\(\,\,\,\,\,\)\({b^t} = {{\text{e}}^{ – 0.12t}}\)

\(b = {{\text{e}}^{ – 0.12}}{\text{ (exact), }}0.887\)    A1     N3

METHOD 2

valid approach     (M1)

eg\(\,\,\,\,\,\)\(\ln (a \times {b^t}) =  – 0.12t + 4.67\)

correct use of log laws for \(\ln (a{b^t})\)     (A1)

eg\(\,\,\,\,\,\)\(\ln a + t\ln b\)

comparing coefficients     (A1)

eg\(\,\,\,\,\,\)\( – 0.12 = \ln b\)

\(b = {{\text{e}}^{ – 0.12}}{\text{ (exact), }}0.887\)    A1     N3

[4 marks]

b.
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