+0

# aritmetic progression

+1
278
8
+302

find the 20th term from the last term of the A.P: 3,8,13,,,,,253

SARAHann  Mar 22, 2017
Sort:

#1
+10657
+1

Common difference between terms is  5       (8-3 = 5      13-8 = 5 etc)

this is of the form    3 + 5 (n-1)

3 + 5n -5 = 5n-2         where n= number of the term

20th term       5(20)-2 = 98

ElectricPavlov  Mar 22, 2017
#2
+302
+1

can we find the answer by reversing the A.P?

SARAHann  Mar 22, 2017
#3
+79818
+1

Good idea.....!!!

The last term  is   253

And the common difference in terms is 5

So......the 20th term from the last  =

253 - 5(n - 1) =

253 - 5(20 - 1)  =

253 - 5(19)  =

158

CPhill  Mar 22, 2017
#4
+302
+1

Is d= -5 or 5?

then if it's 5 shouldn't it  be 253+(n-1)*5?

SARAHann  Mar 22, 2017
#5
+79818
+1

I suppose that you're technically correct......the common difference is -5 [ we are subtracting 5 each time ]

CPhill  Mar 22, 2017
#8
+302
+1

OK I get it! thx!

SARAHann  Mar 25, 2017
#6
+10657
+1

I will continue the way I started

we have 5n-2  gives the nth term

253 = 5n-2

255 = 5n

yields n = 51st term for 253      20 terms before this would be 31st term

5(31)-2 = 153   = 20 terms before 253

I think Chris answer may be the 19th before 253

if you use his equation    the tem that is ONE before would be 253 - 5(1-1) = 253    which would be incorrect.

ElectricPavlov  Mar 22, 2017
#7
+79818
+1

OK.....thanks, EP....

CPhill  Mar 22, 2017

### 28 Online Users

We use cookies to personalise content and ads, to provide social media features and to analyse our traffic. We also share information about your use of our site with our social media, advertising and analytics partners.  See details