Skip main navigation

It’s your turn on growth and decay problems

It's your turn on Growth and decay problems
12.4
Hello. Welcome back. It’s your turn now. But before solving the exercises, let me remind you how logarithms are so powerful in order to solve equations in the exponent. For instance, if you wish to solve an equation of the form a to the x equals to b and the unknown is x, well, it is enough to take on both sides of the equality the logarithm to base anything you want, for instance, a, and you get the solution. For instance, if we apply the logarithm to base a on both sides of the equality, we get logarithm to base a of a to the x equals to logarithm to base a of b. But this is exactly x.
60.6
So we get immediately x equals to logarithm to base a of b. And this is the solution to our equation. Now, in Exercise 1, we’ve got a capital of 8 pounds that is invested with a compound interest of 5% per year. This means that after n years, we get the initial capital, 8000 pounds, multiplied by 1 plus 5 divided by 100 to the n. And we want this to be equal to 10000. So the equation is this quantity– find n in such a way that this quantity equals 10000. Well, this is equivalent to 1 plus 5 divided by 100 to the n equals 10 divided by 8, which is 5 over 4.
130.4
And so we get 1.05 to the n equals, well, 5/4. And then in order to get n, we apply a logarithm, any logarithm. But the most convenient is probably logarithm to base 1.05, because it gives just exactly n when we apply it to 1.05 to the n. So this is equivalent to logarithm to base 1.05 of 1.05 to the n equals to logarithm to base 1.05 of 5/4. Well, this is exactly n.
179.4
And so we get n equals to logarithm to base 1.05 of 5/4. Now, if you want to compute it, we can write that the logarithm to base 1.05 of 5/4 is expressed in terms of the natural logarithm as natural logarithm 5/4 divided by natural logarithm of 1.05. And this gives more or less 4.57 and so on. And this ends Exercise 1.
223.3
In Exercise 2, we are facing a problem concerning the mass of a radioactive substance, in particular the polonium. Actually, remind me of spy story. I don’t know why. Well, the mass is expressed as the initial mass, m0, times 2 to the minus t over 140. And they ask us how many days– this is expressed, t in days. So t is expressed in terms of days. And they ask us how many days does it take to decay, for the mass to decay to half of the initial mass. So we want to find t in such a way that m of t equals m0 divided by 2.
280.8
So the equation is m0, 2 to minus t to 140 equals to m0 divided by 2. We simplify and we get 2 times 2 minus t divided by 140 equals to 1, which is 2 to 1 minus t divided 140 equals to 1. 1 is also 2 to 0. And so this is equivalent since the 2 to the x is an injective function, this is– and it takes the value 1 just in 0, this is equivalent to 1 minus t over 140 equals to 0. That is, t equals 140 days.
340.8
And this answers Exercise 2.

Do your best in trying to solve the following problems. In any case some of them are solved in the video and all of them are solved in the pdf file below.

Exercise 1.

A capital of 8 000 pounds is invested with a given annual compound interest of (5%). How many years and months does it take to reach 10 000 pounds? Express the result in decimal form (ex: 6.87)

Exercise 2.

The mass of the Polonium, a radioactive substance, is expressed at time (t) by the formula [m(t)=m_02^{-t/140},] where (m_0) is the initial mass. How many days does it take to decay to half its original mass?

This article is from the free online

Advanced Precalculus: Geometry, Trigonometry and Exponentials

Created by
FutureLearn - Learning For Life

Our purpose is to transform access to education.

We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.

We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.
You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.

Learn more about how FutureLearn is transforming access to education