Question

How do i calculate? All the way to s_75. It’s so long

Answer 1

Answer:

  -675

Step-by-step explanation:

The sum can be broken into parts that you know. Here, one of those parts is the sum of numbers 1 to n. That sum is given by n(n+1)/2.

$\displaystyle S_{75}=\sum\limits_{n=1}^{75}{(67-2n)}=\sum\limits_{n=1}^{75}{67}-2\sum\limits_{n=1}^{75}{n}\\\\=75\cdot 67-2\dfrac{75(75+1)}{2}=75(67-76)\\\\=\bf{-675}$

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Another way to do this is to realize the sequence of numbers is an arithmetic sequence with a first term of 65 and a last term of 67-2·75 = -83.

The sum of an arithmetic sequence is found by multiplying the number of terms by their average value. Their average value is the average of the first and last terms.

The average value of those 75 terms is (65 +(-83))/2 = -9, so their sum is ...

  75(-9) = -675