HELP! Find the 10th partial sum of...

Question

anonymous · Answer

attachment

anonymous · Answer

Do you want to fin
$$
\sum_{i=3}^{10} -5 i +2=-244 \or\
\sum_{i=3}^{12} -5 i +2=-355
$$

anonymous · Answer

Tell me which one and I will show you how to get the answer

anonymous · Answer

I guess the 10th since I have to find the 10th partical sum? The sign above sigma is infinity.

anonymous · Answer

10 th  means summing 10 terms which make the second choice more acceptable

anonymous · Answer

Do you know the formula
$$
\sum_{i=1}^n i= \frac {n(n+1)}2\Hence
\
\sum_{i=3}^n i= \frac {n(n+1)}2-( 1+2)\
\sum_{i=3}^{12} i= \frac {12(13)}2-( 1+2)=78 -3 =75\
$$
The rest should be easy

anonymous · Answer

I know a little; is the next step for 153 = 75 ?

anonymous · Answer

$$

-5\sum_{i=3}^{12} i =-5(75)=-375\
\sum_{i=3}^{12} 2= 2 (10)=20\

-375+20=-355
$$

anonymous · Answer

Check this link out http://www.wolframalpha.com/input/?i=sum+%28+-5i+%2B2%29+i+from+3+to+12

anonymous · Answer

Do you understand now?

anonymous · Answer

Yes, thanks:) That's cool how you can type it into that link.:o

anonymous · Answer

Just go to http://wolframalpha.com and type in the window and hit return