I am really stuck on a question..
(Q) Find x from the following:
34.56, x, 24, 20.
The asnwer is 28.80. I don't know how to get that. The two fomula you can use are Tn = a+(n-1)d or Sn= n/2 [2a+(n-1)d]
Thanks if you can show me how to do it!
Hello dizzycarlyThe formulae you quote are related to Arithmetic Progressions - where the differences between consecutive terms are equal. The difference between and is , and obviously this can't be the difference between and , and and .
So we must look elsewhere.
In fact, this is a Geometric Progression, where the ratio of consecutive terms is constant. You do it like this:
The ratio of to is . In other words, we must multiply by to make . If this sequence is a GP, we shall be able to multiply by to get , and then multiply by to get . And this works!
Try it out: , and .
So .
(In fact, the common ratio of a GP is the ratio that will take you from left to right through the sequence of numbers; in this case that's the ratio the other way round; i.e. , or .)
Grandad