Geometric Sequence Phrase Issues

This lesson will present you find out how to remedy quite a lot of geometric sequence phrase issues.

Instance #1:

The inventory’s worth of an organization shouldn’t be doing effectively currently. Suppose the inventory’s worth is 92% of its earlier worth every day. What’s the inventory’s worth after 10 days if the inventory was price $2500 proper earlier than it began to go down?  

Answer

To unravel this downside, we’d like the geometric sequence method proven beneath.

a_n = a₁ × r^{(n – 1)}

a₁ = authentic worth of the inventory  = 2500

a₂ = worth of the inventory after 1 day

a₁₁ = worth of the inventory after 10 days

r = 0.92

a₁₁ = 2500 × (0.92)^{(11 – 1)}

a₁₁ = 2500 × (0.92)¹⁰

a₁₁ = 2500 × 0.434

a₁₁ = $1085

The inventory’s worth is about 1085 {dollars}.

Instance #2:

The third time period of a geometrical sequence is 45 and the fifth time period of the geometric sequence is 405. If all of the phrases of the sequence are optimistic numbers, discover the fifteenth time period of the geometric sequence.

Answer

To unravel this downside, we’d like the geometric sequence method proven beneath.

a_n = a₁ × r^{(n – 1)}

Discover the third time period

a₃ = a₁ × r^{(3 – 1)}

a₃ = a₁ × r²

For the reason that third time period is 45,  45 = a₁ × r² (equation 1)

Discover the fifth time period

a₅ = a₁ × r^{(5 – 1)}

a₅ = a₁ × r⁴

For the reason that fifth time period is 405,  405 = a₁ × r⁴ (equation 2)

Divide equation 2 by equation 1.

(a₁ × r⁴) / (a₁ × r²) = 405 / 45

Cancel a₁ since it’s each on prime and on the backside of the fraction.

r⁴ / r² = 9

r² = 9

r = ±√9

r = ±3

Use r  = 3, and equation 1 to discover a₁

45 = a₁ × (3)²

45 = a₁ × 9

a₁ = 45 / 9 = 5

a_n = a₁ × r^{(n – 1)}

Allow us to now discover a₁₅

a₁₅ = 5 × (3)^{(15 – 1)}

a₁₅ = 5 × (3)¹⁴ 

a₁₅ = 5 × 4782969 

a₁₅ =  23914845

Difficult geometric sequence phrase issues

Instance #3:

Suppose that the magnification of a PDF file on a desktop laptop is elevated by 15% for every degree of zoom. Suppose additionally that the unique size of the phrase “January” is 1.2 cm. Discover the size of the phrase “January” after 6 magnifications.

Answer

To unravel this downside, we’d like the geometric sequence method proven beneath.

a_n = a₁ × r^{(n – 1)}

a₁ = authentic size of the phrase  = 1.2 cm

a₂ = size of the phrase after 1 magnification

a₇ = size of the phrase after 6 magnifications

r = 1 + 0.15 = 1.15

n = 7

a₇ = 1.2 × (1.15)^{(7 – 1)}

a₇ = 1.2 × (1.15)⁶

a₇ = 1.2 × (1.15)⁶

a₇ = 1.2 × 2.313

a₇ = 2.7756

After 6 magnifications, the size of the phrase “January” is 2.7756 cm.

Day 1: a₁ = 1.2

Day 2: a₂ = 1.2 + 1.2(0.15) = 1.2(1 + 0.15)

Day 3: a₃ = 1.2(1 + 0.15) + [1.2(1 + 0.15)]0.15 = 1.2(1 + 0.15)(1 + 0.15) = 1.2(1 + 0.15)²

Day 7: a₇ = 1.2(1 + 0.15)⁶

Instance #4

Suppose that you really want a decreased copy of {a photograph}. The precise size of the {photograph} is 10 inches. If every discount is 64% of the unique, what number of reductions, will shrink the {photograph} to 1.07 inches.

Answer

a_n = a₁ × r^{(n – 1)}

a₁ = authentic size of the {photograph}  = 10 inches

a₂ = size of the {photograph} after 1 discount

a_n = 1.07

r = 0.64

n = variety of reductions = ?

1.07 = 10 × (0.64)^{(n – 1)}

Divide either side by 10

1.07 / 10 = [10 × (0.64)^{(n – 1)}] / 10

0.107 = (0.64)^{(n – 1)}

Take the pure log of either side of the equation.

ln(0.107) = ln[(0.64)^{(n – 1)}]

Use the energy property of logarithms.

ln(0.107) = (n – 1)ln(0.64)

Divide either side of the equation by ln(0.64)

ln(0.107) / ln(0.64) = (n – 1)ln(0.64) / ln(0.64)

n – 1 = ln(0.107) / ln(0.64)

Use a calculator to seek out ln(0.107) and ln(0.64)

n – 1 = -2.23492644452 -0.44628710262

n – 1 = 5.0078

n = 1 + 5.0078

n = 6.0078

Due to this fact, you’ll need 6 reductions.