The Student Room Group

C1 Help Plz!

Im doin a paper and stuck on this one, im not very good at these :P

I will rep the answerer!!

In the first month after opening, a mobile phone shop sold 280 phones. A model for future trading assumes that sales will increase by x phones per month for the next 35 months, so that (280 + x) phones will be sold in the second month, (280 + 2x) in the third month, and so on.

Using this model with x = 5, calculate

(a) (i) the number of phones sold in the 36th month,

(ii) the total number of phones sold over the 36 months.


The shop sets a sales target of 17 000 phones to be sold over the 36 months.

Using the same model,

(b) find the least value of x required to achieve this target.
a) i) 36th term = a + d(n - 1)
280 + 25(5) = £455

a) ii) a = 280, n = 36, d = 5

Sn = n/2(2a + (n - 1)d)

Sn = 18(560 + 35(5)) = 13230

b)

Sn = 17000, a = 280, n = 36

17000 = n/2(560 + 35d)
17000 = 10080 + 630d
630d = 6920, d = 10.9841
Least value of of x is rounded to 11.
Seems a bit ridiculous that this is on a non-calc paper.... 6920/630