What is the sum of the first five terms of a geometric series with a1 = 20 and r = 1/4?

1 answer:

4 0

Sum of the first n terms of a geometric sequence is Sn = a(1 - r^n)/(1 - r)
Sn = 20(1 - (1/4)^5)/(1 - 1/4) = 20(1 - 1/1024)/(3/4) = 20(1023/1024)/(3/4) = 26 41/64

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The speed of the current in a creek is 5 mph. A person clan kayak 3 mi upstream in the same time that it takes him to kayak 13 m

Brut [27]

Answer:

Step-by-step explanation:

0) the basic formula is: L=v*t, where L - distance, v - speed/velocity; t - time;

1) if the person's speed in still water is 'v' and the speed of water is 5 (according to the condition), then the upstream speed is 'v-5' and the downstream speed is 'v+5';

2) according to the condition the upstream time and the downstream time are the same, it means t₁=t₂=t, where t₁=upstream time and t₂=downstream time;

3) according to the items above it is possible to make up the equation of the upstream travel: t(v-5)=3; ⇒ t=3/(v-5);

4) according to the items above it is possible to make up the equation of the downstream travel: t(v+5)=13; ⇒ t=13/(v+5);

5) if t=3/(v-5) and t=13/(v+5), then

$\frac{3}{v-5} =\frac{13}{v+5}; \ => \ v=8.$