Arithmetic Progression Difficulty

[Pokeyguy20]

So...


Someone invests £1000 in an account offering an interest rate of 5% each year. How many years will it take to get £1500.


I used the equation Sn=n/2[2a+(n-1)d] and I have made d=1.05, a=1000 and Sn=1500.


I got a quadratic equation which came out at about 1.5 and -2000 which can't be right.


So have I made a glaring error or used the wrong equation?


Any help/advice would be great.


Thanks

 

[Deleted member 4993]

So...


Someone invests £1000 in an account offering an interest rate of 5% each year. How many years will it take to get £1500.


I used the equation Sn=n/2[2a+(n-1)d] and I have made d=1.05, a=1000 and Sn=1500.


I got a quadratic equation which came out at about 1.5 and -2000 which can't be right.


So have I made a glaring error or used the wrong equation?


Any help/advice would be great.


Thanks

You are Not dealing with arithmetic progression here. This is Geometric progression:

a_n = a₁ * r^(n-1)

where:

a₁ = 1000 and

r = 1.05