Ok so this week we are on chapter 13. This chapter is kinda sorta easy for right now because we basically use the same formulas which are arithmetic and geometric. Im going to explain the formulas for series.
Arithmetic series: Sn=(n(t1+tn))/2
Geometric series: Sn=(t1(1-r^n))/1-r
Ex: Find the sum of the first 25 terms of the series...11+14+17+20+...
first you find tn which is t25 and use the arithmetic formula which is t1+(n-1)d so it will be 11+(24)(3)=83.
Then you plug in the arithmetic series formula: Sn=25(11+83)/2=1175 and that is your answer.
The only thing i have problems with is what do i do with n when they dont tell me how many terms there are?
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Do you remember when B-rob was explaining all of this? She would do an example and one of the first things she did when given the sequence...
ReplyDelete3,4,5,6,...
she would lable
n=1 t1=3
n=2 t2=4
n=3 t3=5
n is your term number. not the number in the series. t1/2/3... is the actual number in the series.
N is not a number of terms, N is your term number. Its just there to help to understand what term number is in question, not to evaulate the amount of them.
ReplyDeleteif the problem does not state to what term you are looking for then the question is probably asking for the place number of a specific number in a series
ReplyDeletethink of n as an address n stands for where in a series a specific number lives.
for example you want to know the place in the series for 12
12 would be yout tn
looking at the series
2,4,6,8,10,12
therefore after using the formula you would get n= 6 because 12 would be the 6th term of the series