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C1 Mixed Exercise 6H

Not particularly strong on this topic. Struggling on an easy question, don't know how I'm meant to go about solving it:

The rth term in a sequence is (r+3)(r-4). Find the value of r for the term that has the value 78

Don't really know how to start it other than putting in the 78. Can anyone shed some light on how I'm supposed to do this. Thanks for any help.
Reply 1
Avatar for TeeEm
TeeEm
19
Original post by GingerJ
Not particularly strong on this topic. Struggling on an easy question, don't know how I'm meant to go about solving it:

The rth term in a sequence is (r+3)(r-4). Find the value of r for the term that has the value 78

Don't really know how to start it other than putting in the 78. Can anyone shed some light on how I'm supposed to do this. Thanks for any help.


expand and factorize
or
inspection(trial and error if you leave it as it is)
Reply 2
Avatar for Notnek
Notnek
21
Original post by GingerJ
Not particularly strong on this topic. Struggling on an easy question, don't know how I'm meant to go about solving it:

The rth term in a sequence is (r+3)(r-4). Find the value of r for the term that has the value 78

Don't really know how to start it other than putting in the 78. Can anyone shed some light on how I'm supposed to do this. Thanks for any help.

The nth term of a sequence e.g. 2n+1, gives you the term of the sequence in the nth position.

So the first term is 2 x 1 + 1 etc.


So if the rth term (same concept as nth term) is (r+3)(r-4) then e.g. the 2nd term will be

(2+3)(2-4) = -10

This is -10 not 78. Which term will be 78?

You could use trial and error. Or set up and equation and solve it.
Reply 3
Avatar for Josef314
Josef314
(r+3)(r-4) = 78
so simply :
r^2 -r -12 = 78
r^2 -r - 90 = 0 [ rearranging]
r^2 + 10r - 9r - 90 = 0 [simple factorising]
r(r+10)-9(r+10) = 0 [factorising]
(r-9)(r+10) = 0 [creating brakets]
so r = 9 or r = -10 [finding r]

I think this is how you are supposed to do it ...
Reply 4
Avatar for davros
davros
16
Original post by Josef314
(r+3)(r-4) = 78

STUFF

I think this is how you are supposed to do it ...


Please don't post full solutions - it's against the rules of the forum :smile:
Reply 5
Avatar for Josef314
Josef314
Original post by davros
Please don't post full solutions - it's against the rules of the forum :smile:


Oh, alright my bad didn't know !

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