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Otrada [13]
3 years ago
11

Find S15 for the geometric series 72 + 12 + 2 + +…

Mathematics
1 answer:
Sedbober [7]3 years ago
6 0
S_{15}=72+12+\cdots+\dfrac1{181398528}+\dfrac1{1088391168}
S_{15}=72\left(\dfrac16\right)^0+72\left(\dfrac16\right)^1+\cdots+72\left(\dfrac16\right)^{13}+72\left(\dfrac16\right)^{14}
S_{15}=72\displaystyle\sum_{i=1}^{15}\left(\frac16\right)^{i-1}

\dfrac16S_{15}=72\displaystyle\sum_{i=1}^{15}\left(\frac16\right)^i

\implies S_{15}-\dfrac16S_{15}=72\displaystyle\sum_{i=1}^{15}\bigg(\left(\dfrac16\right)^{i-1}-\left(\dfrac16\right)^i\bigg)
\dfrac56S_{15}=72\bigg(\left(1-\dfrac16\right)+\left(\dfrac16-\dfrac1{6^2}\right)+\cdots+\left(\dfrac1{6^{13}}-\dfrac1{6^{14}}\right)+\left(\dfrac1{6^{14}}-\dfrac1{6^{15}}\right)\bigg)
\dfrac56S_{15}=72\left(1-\dfrac1{6^{15}}\right)
S_{15}=\dfrac{432}5\times\dfrac{6^{15}-1}{6^{15}}=86.4
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1) x^2=x\cdot x, and x(x+3)=x^2+3x. Subtracting this from the numerator gives a remainder of

(x^2-13x-48)-(x^2+3x)=-16x-48

-16x=-16\cdot x, and -16(x+3)=-16x-48. Subtracting this from the previous remainder gives a new remainder of

(-16x-48)-(-16x-48)=0

This means that

\dfrac{x^2-13x-48}{x+3}=x-16

2) 3x^3=3x^2\cdot x, and 3x^2(x+2)=3x^3+6x^2. Subtracting this from the numerator gives a remainder of

(3x^3-x^2-7x+6)-(3x^3+6x^2)=-7x^2-7x+6

-7x^2=-7x\cdot x, and -7x(x+2)=-7x^2-14x. Subtracting this from the previous remainder gives a new remainder of

(-7x^2-7x+6)-(-7x^2-14x)=7x+6

7x=7\cdot x, and 7(x+2)=7x+14. Subtracting this from the previous remainder gives a new remainder of

(7x+6)-(7x+14)=-8

This means that

\dfrac{3x^3-x^2-7x+6}{x+2}=3x^2-7x+7-\dfrac8{x+2}

3) x+2 will be a factor of x^3+3x^2-10x-24 if dividing the latter by x+2 leaves a remainder of 0.

x^3=x^2\cdot x, and x^2(x+2)=x^3+2x^2. Subtracting this from the numerator gives a remainder of

(x^3+3x^2-10x-24)-(x^3+2x^2)=x^2-10x-24

x^2=x\cdot x, and x(x+2)=x^2+2x. Subtracting this from the previous remainder gives a new remainder of

(x^2-10x-24)-(x^2+2x)=-12x-24

-12x=-12\cdot x, and -12(x+2)=-12x-24. Subtracting this from the previous remainder gives a new remainder of

(-12x-24)-(-12x-24)=0

and since the remainder is 0, x+2 is indeed a factor of x^3+3x^2-10x-24.

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3 years ago
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