Help me with this:(((

<img src="https://tex.z-dn.net/?f=%24a%2Ba%20r%2Ba%20r%5E%7B2%7D%2B%5Cldots%20%5Cinfty%3D15%24%24a%5E%7B2%7D%2B%28a%20r%29%5E%7B

riadik2000 [5.3K]

Let

$S_n = \displaystyle \sum_{k=0}^n r^k = 1 + r + r^2 + \cdots + r^n$

where we assume |r| < 1. Multiplying on both sides by r gives

$r S_n = \displaystyle \sum_{k=0}^n r^{k+1} = r + r^2 + r^3 + \cdots + r^{n+1}$

and subtracting this from $S_n$ gives

$(1 - r) S_n = 1 - r^{n+1} \implies S_n = \dfrac{1 - r^{n+1}}{1 - r}$

As n → ∞, the exponential term will converge to 0, and the partial sums $S_n$ will converge to

$\displaystyle \lim_{n\to\infty} S_n = \dfrac1{1-r}$

Now, we're given

$a + ar + ar^2 + \cdots = 15 \implies 1 + r + r^2 + \cdots = \dfrac{15}a$

$a^2 + a^2r^2 + a^2r^4 + \cdots = 150 \implies 1 + r^2 + r^4 + \cdots = \dfrac{150}{a^2}$

We must have |r| < 1 since both sums converge, so

$\dfrac{15}a = \dfrac1{1-r}$

$\dfrac{150}{a^2} = \dfrac1{1-r^2}$

Solving for r by substitution, we have

$\dfrac{15}a = \dfrac1{1-r} \implies a = 15(1-r)$

$\dfrac{150}{225(1-r)^2} = \dfrac1{1-r^2}$

Recalling the difference of squares identity, we have

$\dfrac2{3(1-r)^2} = \dfrac1{(1-r)(1+r)}$

We've already confirmed r ≠ 1, so we can simplify this to

$\dfrac2{3(1-r)} = \dfrac1{1+r} \implies \dfrac{1-r}{1+r} = \dfrac23 \implies r = \dfrac15$

It follows that

$\dfrac a{1-r} = \dfrac a{1-\frac15} = 15 \implies a = 12$

and so the sum we want is

$ar^3 + ar^4 + ar^6 + \cdots = 15 - a - ar - ar^2 = \boxed{\dfrac3{25}}$

which doesn't appear to be either of the given answer choices. Are you sure there isn't a typo somewhere?

7 0

2 years ago

The length of a rectangle is 3ft more than it's width. if the area is 28ft^2 find the length and width.

Semmy [17]

What 2 # multiplied together make 28,
7x4
2x14 out of these which one has one side 3ft. more? 7x4

6 0

2 years ago

Mrs. lama marked a price of a cosmetic item 25% above it's cost price. if the cost price of the cosmetic item was 4400, at what

Alexeev081 [22]

Answer:

hi dear I suppose the answer would be 6271$

3 0

2 years ago

Which of the following are situations that can be modeled with a quadratic function? Select all that apply.

Lina20 [59]

Answer:

Option 2 and 5 are correct.

Step-by-step explanation:

We need to tell which one of them is quadratic function.

Option 1 is exponential decay so, it is not quadratic.

Option 2 is quadratic because the diver will take the parabolic shape when jumps.

Option 3 is not quadratic.Option 4 is not quadratic it is linear.

Option 4 is again exponential not quadratic.

Option 5 is quadratic because it takes the parabolic shape again.

4 0

3 years ago

Read 2 more answers

Which of the following fractions is equivalent to -84/-90 in the least common terms?

abruzzese [7]

Answer: 14/15

This is because to get to 14/15 you must divide by 6 but when getting to 42/45 you must divide 2. Therefore 14/15 is your answer because the problem went to the most reduced fraction.

5 0

2 years ago

Help me with this:(((​

Help me with this:(((