How many unit squares are there in the next square in this spiraling sequence?
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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 1 6 9 square (of side 1 3 ) can be expressed as 8 + 5 which is the sum of the previous 2 squares 6 4 and 2 5 .
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 where a is the side length, you can notice that the previous two squares have side lengths 3 4 ( 3 4 2 = 1 1 5 6 ) and 2 1 ( 2 1 2 = 4 4 1 ) . So the side would be 3 4 + 2 1 = 5 5 units. Reapplying the formula for area of a square, 5 5 2 = 3 0 2 5 .
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First, you need to know that this is the Fibonacci Sequence:
1 , 1 , 2 , 3 , 5 … F n = F n − 1 + F n − 2
The only difference is each of a Fibonacci number has been squared:
1 , 1 , 4 , 9 , 2 5 … ( F n ) 2 = ( F n − 1 + F n − 2 ) 2
In this case, F n − 1 are 4 4 1 = 2 1 and F n − 2 are 1 1 5 6 = 3 4
Therefore, the final correct answer is ( 2 1 + 3 4 ) 2 = 5 5 2 = 3 0 2 5