Help with maths question

[Recycle Bin]

[FChaosi_]

u4/u1 = r^3

r^3 = [1/3]/[1/81]

r^3 = 27

r = 3

Graph Sn using sum of a geometric series formula. Find Sn = 40 using graphics calculator. Given that Sn increases as n increases, round to the next n if needed. 

[Recycle Bin]

32 minutes ago, FChaosi_ said:

u4/u1 = r^3

r^3 = [1/3]/[1/81]

r^3 = 27

r = 3

Graph Sn using sum of a geometric series formula. Find Sn = 40 using graphics calculator. Given that Sn increases as n increases, round to the next n if needed. 

thank you!

[astonky]

Or for part (b) you can also start solving it algebraically:

u₁*rⁿ = 40 (establish equation using the formula for geometric equations in the formula booklet)

Rearrange the equation: rⁿ = 40/u₁

Substitute for u₁: rⁿ =40/(1/81)

So: rⁿ= 40*81 which is 3240

Using logarithms:

log₃ 3240 = n

On a non-calculator find out 3 to the power of what number is greater than 3240 using trial and error:

eg. 3⁵ = 243

3⁶ = 729

3⁷ = 2187

3⁸ = 6561

On a calculator just use the log solver to find that n = 8