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1. When Yin attempts to add all the positive integers from 1 through 99, he accidentally skips a number and gets the sum 4909. Which number did Yin skip?

2. Compute the sum of $$$\left(1 - \frac 16\right) + \left(\frac12 - \frac17\right) + \left(\frac13 - \frac18\right) + \cdots + \left(\frac{1}{95} - \frac{1}{100}\right).$$$ Express your answer as a decimal to the nearest tenth.

3. Compute the product $$$(-39 + 13)\cdot(-36 + 12) \cdot(-33 + 11) \cdot(-30 + 10) \cdots (33 -11)\cdot (36 - 12) \cdot (39 - 13).$$$Where the first summand in each factor is increasing by 3 and the second summand is decreasing by 1.

Sep 2, 2021

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1. When Yin attempts to add all the positive integers from 1 through 99, he accidentally skips a number and gets the sum 4909. Which number did Yin skip?

The formula for the sum of numbers 1 + 2 + 3 • • • + n   is (n)(n+1) / 2

(99)(100) / 2 = 4950      If Yin got 4909 he omitted (4950 – 4909) = 41

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Sep 2, 2021
#2
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Thank dude, I also need some help on 2 and 3 but i now fully understand 1!

Guest Sep 2, 2021
#3
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#2:  The positive terms are:

1 + 1/2 + 1/3 + 1/4 + 1/5 + 1/6 + 1/7 + 1/8 + ... + 1/93 + 1/94 + 1/95

The negative terms are:

- 1/6 - 1/7 - 1/8 - ... -1/93 - 1/94 - 1/95 - 1/96 - 1/97 - 1/98 - 1/99 - 1/100

Cancelling the equivalent positive and negative terms, we are left with:

1 + 1/2 + 1/3 + 1/4 + 1/5 - 1/96 - 1/97 - 1/98 - 1/99 - 1/100

Now, it's calculator time!

#3:  If you write out the factors, you have, for one of the factors,  (-12 + 12)  which is 0.

Therefore, the product is 0.

Sep 2, 2021