Spiral Area

How many unit squares are there in the next square in this spiraling sequence?

32 × 32 = 1024 32 \times 32 = 1024 50 × 50 = 2500 50 \times 50 = 2500 64 × 64 = 4096 64 \times 64 = 4096 55 × 55 = 3025 55 \times 55 = 3025

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2 solutions

First, you need to know that this is the Fibonacci Sequence:

1 , 1 , 2 , 3 , 5 F n = F n 1 + F n 2 1, 1, 2, 3, 5\ldots F_n=F_{n-1}+F_{n-2}

The only difference is each of a Fibonacci number has been squared:

1 , 1 , 4 , 9 , 25 ( F n ) 2 = ( F n 1 + F n 2 ) 2 1, 1, 4, 9, 25\ldots {(F_n)}^2=(F_{n-1}+F_{n-2})^2

In this case, F n 1 F_{n-1} are 441 = 21 \sqrt{441}=21 and F n 2 F_{n-2} are 1156 = 34 \sqrt{1156}=34

Therefore, the final correct answer is ( 21 + 34 ) 2 = 5 5 2 = 3025 (21+34)^2=55^2=\large{\boxed{3025}}

Vishnu Bhagyanath
Aug 30, 2015

In the figure, starting from the 3rd square in the spiral, notice that the side of any square is the sum of the sides of the previous 2 squares. For example, the 169 169 square (of side 13 13 ) can be expressed as 8 + 5 8 + 5 which is the sum of the previous 2 squares 64 64 and 25 25 .

Similarly, the side of the required square would be the side of the previous two squares. If you remember that the area of a square is a 2 a^2 where a a is the side length, you can notice that the previous two squares have side lengths 34 ( 3 4 2 = 1156 ) 34 (34^2=1156) and 21 ( 2 1 2 = 441 ) 21 (21^2=441) . So the side would be 34 + 21 = 55 34 + 21 = 55 units. Reapplying the formula for area of a square, 5 5 2 = 3025 55^2 = 3025 .

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