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

∑(3k=126k-7)

Short Answer

Expert verified

The sum of this sequence is 871.

Step by step solution

01

Step 1. Write the given information. 

The sum of sequence:

∑(3k-7)k=126

02

Step 2. Use the properties of sequence.

Using the properties of sequence:

∑(ak-bk)k=1n=∑(ak)k=1n-∑(bk)k=1n&∑(cak)k=1n=c∑akk=1n

So,

∑(3k-7)k=126=∑(3k)k=126-∑(7)k=126

&

∑(3k)k=126=3∑(k)k=126

03

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

Using formula for summation is:

∑kk=1n=1+2+...+n⇒∑kk=1n=n(n+1)2

So,

role="math" localid="1646750162806" 3∑kk=126=3×26(26+1)2⇒3∑kk=126=3×26(27)2⇒3∑kk=126=3×351⇒3∑kk=126=1053

04

Step 4. Use the formula for sum of sequence. 

Using formula for summation is:
∑ck=1n=c+c+...+c⇒∑ck=1n=cn,c-realnumberSo,∑7k=126=7×26⇒∑7k=126=182

05

Step 5. Now, subtract Step 3  from Step 4.

From Step 3,

3∑kk=126=1053

From Step 4,

∑7k=126=182

So,

∑(3k-7)k=126=∑(3k)k=126-∑(3)k=126⇒∑(3k-7)k=126=1053-182⇒∑(3k-7)k=126=871

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