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The third, fifth and eighth terms of an arithmetic progression are consecutive terms of a geometric progression. Given that the first term of the AP is 8, determine the common difference and the common ratio.

Mathematics

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Part 5· 4 steps

The third, fifth and eighth terms of an arithmetic progression are consecutive terms of a geometric progression. Given that the first term of the AP is 8, determine the common difference and the common ratio.

  1. Define the terms of the AP.

    Let the first term of the AP be aa and the common difference be dd.
    Given a=8a = 8.
    The third term is T3=a+2d=8+2dT_3 = a + 2d = 8 + 2d.
    The fifth term is T5=a+4d=8+4dT_5 = a + 4d = 8 + 4d.
    The eighth term is T8=a+7d=8+7dT_8 = a + 7d = 8 + 7d.

  2. Use the property of a geometric progression.

    Since T3,T5,T8T_3, T_5, T_8 are consecutive terms of a GP, the ratio of consecutive terms is constant.

    T5T3=T8T5\frac{T_5}{T_3} = \frac{T_8}{T_5} 8+4d8+2d=8+7d8+4d\frac{8 + 4d}{8 + 2d} = \frac{8 + 7d}{8 + 4d}
  3. Solve the equation for dd.

    Cross-multiply:

    (8+4d)2=(8+2d)(8+7d)(8 + 4d)^2 = (8 + 2d)(8 + 7d) 64+64d+16d2=64+56d+16d+14d264 + 64d + 16d^2 = 64 + 56d + 16d + 14d^2 64+64d+16d2=64+72d+14d264 + 64d + 16d^2 = 64 + 72d + 14d^2

    Subtract 64 from both sides and rearrange terms:

    16d2−14d2+64d−72d=016d^2 - 14d^2 + 64d - 72d = 0 2d2−8d=02d^2 - 8d = 0

    Factor out 2d2d:

    2d(d−4)=02d(d - 4) = 0

    This gives two possible values for dd:
    2d=0  ⟹  d=02d = 0 \implies d = 0
    or
    d−4=0  ⟹  d=4d - 4 = 0 \implies d = 4.

  4. Determine the common ratio for each value of dd.

    Case 1: If d=0d = 0
    The terms of the AP are 8,8,88, 8, 8. The GP terms are 8,8,88, 8, 8.
    The common ratio r=88=1r = \frac{8}{8} = 1.
    Common difference: d=0\boxed{d = 0}, Common ratio: r=1\boxed{r = 1}.

    Case 2: If d=4d = 4
    The terms of the AP are:
    T3=8+2(4)=16T_3 = 8 + 2(4) = 16
    T5=8+4(4)=24T_5 = 8 + 4(4) = 24
    T8=8+7(4)=36T_8 = 8 + 7(4) = 36
    The GP terms are 16,24,3616, 24, 36.
    The common ratio r=2416=32r = \frac{24}{16} = \frac{3}{2}.
    Common difference: d=4\boxed{d = 4}, Common ratio: r=32\boxed{r = \frac{3}{2}}.

Part 6· 4 steps

Two places A and B are 900km apart on the earth surface. If A is due North of B and given that the latitude of A is 5∘N5^\circ N. Find into 1 decimal place the latitude of B. (Take radius of earth to be 6370km).

  1. Identify the given values and the formula for distance along a meridian.

    Distance L=900 kmL = 900 \text{ km}.
    Radius of Earth R=6370 kmR = 6370 \text{ km}.
    Latitude of A = 5∘N5^\circ N.
    The distance along a meridian is given by the formula:

    L=θ360∘×2πRL = \frac{\theta}{360^\circ} \times 2\pi R

    where θ\theta is the difference in latitude in degrees.

  2. Calculate the difference in latitude (θ\theta).

    Substitute the given values into the formula:

    900=θ360∘×2π(6370)900 = \frac{\theta}{360^\circ} \times 2\pi (6370)

    Rearrange to solve for θ\theta:

    θ=900×360∘2π×6370\theta = \frac{900 \times 360^\circ}{2\pi \times 6370} θ=32400012740π≈8.0950∘\theta = \frac{324000}{12740\pi} \approx 8.0950^\circ
  3. Determine the latitude of B.

    Since A is due North of B, B must be South of A.
    Latitude of B = Latitude of A - θ\theta
    Latitude of B = 5∘N−8.095∘=−3.095∘5^\circ N - 8.095^\circ = -3.095^\circ.
    This means B is 3.095∘3.095^\circ South of the equator.

  4. Round the latitude of B to 1 decimal place.

    The latitude of B is 3.1∘S\boxed{3.1^\circ S}.

Part 7· 5 steps

A plot costs 600000 currently. If the rate of appreciation is 15% per annum, find to the nearest whole number the number of years it will take to appreciate to Ksh. 1049404.

  1. Identify the compound appreciation formula and given values.

    The formula for compound appreciation is:

    A=P(1+r)nA = P(1 + r)^n

    Where:
    AA = Final amount = Ksh. 1049404
    PP = Principal amount (current cost) = Ksh. 600000
    rr = Annual rate of appreciation = 15% = 0.15
    nn = Number of years (to be found)

  2. Substitute the values into the formula.

    1049404=600000(1+0.15)n1049404 = 600000(1 + 0.15)^n 1049404=600000(1.15)n1049404 = 600000(1.15)^n
  3. Isolate the term with nn.

    1049404600000=(1.15)n\frac{1049404}{600000} = (1.15)^n 1.74900666...=(1.15)n1.74900666... = (1.15)^n
  4. Solve for nn using logarithms.

    Take the logarithm of both sides:

    log⁡(1.74900666)=nlog⁡(1.15)\log(1.74900666) = n \log(1.15) n=log⁡(1.74900666)log⁡(1.15)n = \frac{\log(1.74900666)}{\log(1.15)}

    Using a calculator:

    n≈0.2427990.060697≈4.0001n \approx \frac{0.242799}{0.060697} \approx 4.0001
  5. Round nn to the nearest whole number.

    The number of years is 4\boxed{4}.

Part 8· 5 steps

A varies partly as B and partly as the square root of B. When B = 4, A = 22 and when B = 9, A = 42. Find A when B = 25.

  1. Write down the variation equation.

    The relationship can be written as:

    A=kB+cBA = kB + c\sqrt{B}

    where kk and cc are constants.

  2. Use the given conditions to form simultaneous equations.

    Condition 1: When B=4,A=22B = 4, A = 22.

    22=k(4)+c422 = k(4) + c\sqrt{4} 22=4k+2c(Equation 1)22 = 4k + 2c \quad \text{(Equation 1)}

    Condition 2: When B=9,A=42B = 9, A = 42.

    42=k(9)+c942 = k(9) + c\sqrt{9} 42=9k+3c(Equation 2)42 = 9k + 3c \quad \text{(Equation 2)}
  3. Solve the simultaneous equations for kk and cc.

    From Equation 1, divide by 2:

    11=2k+c  ⟹  c=11−2k11 = 2k + c \implies c = 11 - 2k

    Substitute this expression for cc into Equation 2:

    42=9k+3(11−2k)42 = 9k + 3(11 - 2k) 42=9k+33−6k42 = 9k + 33 - 6k 42=3k+3342 = 3k + 33

    Subtract 33 from both sides:

    9=3k9 = 3k k=3k = 3

    Now substitute k=3k = 3 back into the expression for cc:

    c=11−2(3)=11−6=5c = 11 - 2(3) = 11 - 6 = 5
  4. Write the complete variation equation.

    Substitute the values of kk and cc into the original equation:

    A=3B+5BA = 3B + 5\sqrt{B}
  5. Find A when B = 25.

    Substitute B=25B = 25 into the complete variation equation:

    A=3(25)+525A = 3(25) + 5\sqrt{25} A=75+5(5)A = 75 + 5(5) A=75+25A = 75 + 25 A=100A = 100

    When B = 25, A is 100\boxed{100}.

Part 9· 5 steps

The floor of a rectangular room measures 4.8m by 3.2m. Estimate the percentage error in the area.

Assumption: The measurements 4.8m and 3.2m are rounded to one decimal place. This means the maximum possible error in each measurement is ±0.05 m\pm 0.05 \text{ m}.

  1. Calculate the nominal area.

    Nominal length L=4.8 mL = 4.8 \text{ m}.
    Nominal width W=3.2 mW = 3.2 \text{ m}.
    Nominal area A=L×W=4.8 m×3.2 m=15.36 m2A = L \times W = 4.8 \text{ m} \times 3.2 \text{ m} = 15.36 \text{ m}^2.

  2. Determine the maximum and minimum possible values for length and width.

    Maximum length Lmax=4.8+0.05=4.85 mL_{max} = 4.8 + 0.05 = 4.85 \text{ m}.
    Minimum length Lmin=4.8−0.05=4.75 mL_{min} = 4.8 - 0.05 = 4.75 \text{ m}.
    Maximum width Wmax=3.2+0.05=3.25 mW_{max} = 3.2 + 0.05 = 3.25 \text{ m}.
    Minimum width Wmin=3.2−0.05=3.15 mW_{min} = 3.2 - 0.05 = 3.15 \text{ m}.

  3. Calculate the maximum and minimum possible areas.

    Maximum possible area Amax=Lmax×Wmax=4.85 m×3.25 m=15.7625 m2A_{max} = L_{max} \times W_{max} = 4.85 \text{ m} \times 3.25 \text{ m} = 15.7625 \text{ m}^2.
    Minimum possible area Amin=Lmin×Wmin=4.75 m×3.15 m=14.9625 m2A_{min} = L_{min} \times W_{min} = 4.75 \text{ m} \times 3.15 \text{ m} = 14.9625 \text{ m}^2.

  4. Calculate the maximum absolute error in the area.

    The maximum absolute error is the larger of (Amax−A)(A_{max} - A) and (A−Amin)(A - A_{min}).
    Amax−A=15.7625−15.36=0.4025 m2A_{max} - A = 15.7625 - 15.36 = 0.4025 \text{ m}^2.
    A−Amin=15.36−14.9625=0.3975 m2A - A_{min} = 15.36 - 14.9625 = 0.3975 \text{ m}^2.
    The maximum absolute error ΔA=0.4025 m2\Delta A = 0.4025 \text{ m}^2.

  5. Calculate the percentage error.

    Percentage error = Maximum absolute errorNominal Area×100%\frac{\text{Maximum absolute error}}{\text{Nominal Area}} \times 100\%
    Percentage error = 0.4025 m215.36 m2×100%≈2.61914%\frac{0.4025 \text{ m}^2}{15.36 \text{ m}^2} \times 100\% \approx 2.61914\%

    Rounding to two decimal places:
    The percentage error in the area is approximately 2.62%\boxed{2.62\%}.

Part 10· 3 steps

There are two grades of tea, grade A and grade B. Grade A costs Ksh. 80 per kg and grade B costs Sh. 60 per kg. In what ratio must the two be mixed in order to produce a blend worth Sh. 75 per kg.

  1. Define variables for the quantities of each grade of tea.

    Let xx be the quantity (in kg) of Grade A tea.
    Let yy be the quantity (in kg) of Grade B tea.

  2. Set up an equation based on the total cost and total quantity.

    Cost of xx kg of Grade A tea = 80x80x.
    Cost of yy kg of Grade B tea = 60y60y.
    Total cost of the mixture = 80x+60y80x + 60y.
    Total quantity of the mixture = x+yx + y.
    The desired price of the blend is Sh. 75 per kg.
    So, the average cost per kg is 75:

    80x+60yx+y=75\frac{80x + 60y}{x + y} = 75
  3. Solve the equation for the ratio xy\frac{x}{y}.

    Multiply both sides by (x+y)(x + y):

    80x+60y=75(x+y)80x + 60y = 75(x + y) 80x+60y=75x+75y80x + 60y = 75x + 75y

    Rearrange the terms to group xx terms and yy terms:

    80x−75x=75y−60y80x - 75x = 75y - 60y 5x=15y5x = 15y

    Divide both sides by 5y5y to find the ratio xy\frac{x}{y}:

    xy=155\frac{x}{y} = \frac{15}{5} xy=31\frac{x}{y} = \frac{3}{1}

    The ratio of Grade A to Grade B is 3:1\boxed{3:1}.

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