Question

you have some marbles. when you make a shape of an equilateral triangle, you either have 5 extra, or you need 10 more to complete the shape, how many marbles do you have? how long is one side of your triangle?
I’m giving brainliest to correct answer!

1. 110 marbles

side 14

Step-by-step explanation:

Draw triangles using marbles as the sides.

If the side is 2 marbles long, the triangle uses 3 marbles. 1 + 2 = 3

If the side is 3 marbles long, the triangle uses 6 marbles. 1 + 2 + 3 = 6

If the side is 4 marbles long, the triangle uses 10 marbles. 1 + 2 + 3 + 4 = 10

If the side is n marbles long, the triangle uses n(n + 1)/2 marbles.

These numbers are called triangular numbers.

We are looking for the side, n.

n(n + 1)/2 + 5 = (n + 1)(n + 2)/2 – 10

n^2 + n + 10 = n^2 + 3n + 2 – 20

n + 10 = 3n – 18

-n = -14

n = 14

For n = 14

n(n + 1)/2 =

= 14(15)/2

= 105

For n = 15

n(n + 1)/2 =

= 15(16)/2

= 120

105 + 5 = 120 – 10

You have 110 marbles. If you had 5 fewer, 105, you’d have a triangle with sides 14. If you had 10 more, 120, you’d have a triangle with sides 15. Since you have 110, you have 5 too many for side 14 and 10 too few for side 15.

The sides of the triangle is 14.