Question

Hola ayudenme porfa se trata de progresiones aritmeticas 1Un niño compra figuras para su album; el primer dia 1 figura ; segundo dia,3 figuras;el tercer dia, 5 figuras y asi sucesivamente. Si en total compro 225 figuras ¿Cuantos dias compro figuras?

Answer 1

Answer: 16 days.

Explanation:

I will answer this in English.

An aritmetic series works in the next way.

if the first therm is A.

Then the second therm is A + r

the third therm is A + r + r

and so on.

Where A is the initial value and r is a constant.

Here the first term is A = 1

The second term is 3, we can find the value of r as:

3 - 1 = r = 2

Then our series is:

Kₙ₊₁ = Kₙ + 2

where K₁ = 1

We know that in total he bought 225 figures, then we have the sumation:

∑Kₙ

and we want to find the value of n.

the sum of the first n terms is:

Sn = (n/2)*(K₁ + Kₙ)

where Kₙ = 1 + n*2 and K₁ = 1

so we must solve:

225 = (n/2)*(1 + 1 + n*2) = n + n^2

So we must solve the quadratic equation:

n^2 + n - 225 = 0

the solutions are:

`n = (-1 + -√(1^2 + 4*225) )/(2) = (-1 +-30)/(2)`

So we have two solutions:

n = 29/3 = 14-.5

n = -31/2 = -15.5

We must select the positive solution, because n can be only positive, and we should round it to the next whole number, that is 16.

So he bought figures for 16 days.