Hello! Need help to help my daughter! I have attached a picture of the question. We know the answer. Just not how to solve.

Hharouff

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2B9F66D5-F604-4F66-9D09-083C4C3F0EA8.jpeg
 

Harry_the_cat

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Best to set up a table of values and look for a pattern.
Look at stage 1 and compare to stage 2:
Can you see that in stage 2, 3 more triangles are removed, one on each side of the triangle removed in stage 1?
Now look and compare stage 2 to stage 3:
Again, for each of the triangles removed in stage 2, there are three more removed around it (ie 3x3 = 9) removed in stage 3
For stage 4, each of these 9 will have 3 removed from around it, ie 9x3 =27 removed.
The "triangles removed column" are powers of 3.

Note that the triangles removed = 3 ^ (stage number -1)

So, the number of triangles removed in stage 21 will be 3^20.

Stage numbertriangles removed
11 = 3^0
23 = 3^1
39 = 3^2
427 = 3^3
 

Hharouff

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Thank you!
 

Dr.Peterson

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Your answer is 3^20, the number removed at the 21st stage. What you are asked for is the total number removed through the 21st stage.

Do you know the sum of a geometric series?
 

Hharouff

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Hharouff

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We solved. Thank you. She was forgetting to divide by 2! Thank you everyone for your help!
 
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