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In Problems 71-82, find the sum of each sequence.

∑k=840(-3k)

Short Answer

Expert verified

The sum of this sequence is -2376.

Step by step solution

01

Step 1. Write the given information. 

The sum of sequence:

∑k=840(-3k)

02

Step 2. Use the property of sequences.

Using the property of sequences:∑(cak)k=1n=c∑akk=1nSo,∑(-3k)k=1n=-3∑kk=1n

03

Step 3. Use the formula for sum of sequences of n real numbers 

∑kk=840=∑kk=140-∑kk=17⇒∑kk=840=40(40+1)2-7(7+1)2⇒∑kk=840=20×41-7×4⇒∑kk=840=820-28⇒∑kk=840=792

04

Step 4. Use the property of sequences from Step 2.

Using the property of sequences gives:

∑kk=840=792⇒(-3)∑kk=840=-3×792⇒(-3)∑kk=840=-2376

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