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dimulka [17.4K]
2 years ago
15

A sum is invested at 4% continuous interest. This means that its value grows exponentially with k equaling the decimal rate of i

nterest. Find, to the nearest tenth of a year, the time required for the investment to double in value.
Mathematics
1 answer:
Zanzabum2 years ago
4 0

Answer:

T = 72/4 = 18years

T = 20years (to the nearest tenth)

Step-by-step explanation:

Using the rule of 72, which is used to estimate the number of years for a given investment to double at a given interest rate.

Doubling time = 72/interest rate

T = 72/r

Rate r (in percentage) = 4%

Time T (in years)

T = 72/4 = 18years

T = 20years (to the nearest tenth)

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Find the two intersection points
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Answer:

Our two intersection points are:

\displaystyle (3, -2) \text{ and } \left(-\frac{53}{25}, \frac{46}{25}\right)

Step-by-step explanation:

We want to find where the two graphs given by the equations:

\displaystyle (x+1)^2+(y+2)^2 = 16\text{ and } 3x+4y=1

Intersect.

When they intersect, their <em>x-</em> and <em>y-</em>values are equivalent. So, we can solve one equation for <em>y</em> and substitute it into the other and solve for <em>x</em>.

Since the linear equation is easier to solve, solve it for <em>y: </em>

<em />\displaystyle y = -\frac{3}{4} x + \frac{1}{4}<em />

<em />

Substitute this into the first equation:

\displaystyle (x+1)^2 + \left(\left(-\frac{3}{4}x + \frac{1}{4}\right) +2\right)^2 = 16

Simplify:

\displaystyle (x+1)^2 + \left(-\frac{3}{4} x  + \frac{9}{4}\right)^2 = 16

Square. We can use the perfect square trinomial pattern:

\displaystyle \underbrace{(x^2 + 2x+1)}_{(a+b)^2=a^2+2ab+b^2} + \underbrace{\left(\frac{9}{16}x^2-\frac{27}{8}x+\frac{81}{16}\right)}_{(a+b)^2=a^2+2ab+b^2} = 16

Multiply both sides by 16:

(16x^2+32x+16)+(9x^2-54x+81) = 256

Combine like terms:

25x^2+-22x+97=256

Isolate the equation:

\displaystyle 25x^2 - 22x -159=0

We can use the quadratic formula:

\displaystyle x = \frac{-b\pm\sqrt{b^2-4ac}}{2a}

In this case, <em>a</em> = 25, <em>b</em> = -22, and <em>c</em> = -159. Substitute:

\displaystyle x = \frac{-(-22)\pm\sqrt{(-22)^2-4(25)(-159)}}{2(25)}

Evaluate:

\displaystyle \begin{aligned} x &= \frac{22\pm\sqrt{16384}}{50} \\ \\ &= \frac{22\pm 128}{50}\\ \\ &=\frac{11\pm 64}{25}\end{aligned}

Hence, our two solutions are:

\displaystyle x_1 = \frac{11+64}{25} = 3\text{ and } x_2 = \frac{11-64}{25} =-\frac{53}{25}

We have our two <em>x-</em>coordinates.

To find the <em>y-</em>coordinates, we can simply substitute it into the linear equation and evaluate. Thus:

\displaystyle y_1 = -\frac{3}{4}(3)+\frac{1}{4} = -2

And:

\displaystyle y _2 = -\frac{3}{4}\left(-\frac{53}{25}\right) +\frac{1}{4} = \frac{46}{25}

Thus, our two intersection points are:

\displaystyle (3, -2) \text{ and } \left(-\frac{53}{25}, \frac{46}{25}\right)

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We conclude that the solution to the given linear equation is x = 15/13.

<h3>How to solve that equation for x?</h3>

Here we have the linear equation:

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