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To calculate $41^2$, David mentally figures the value $40^2$ and adds 81. David subtracts a number from $40^2$ to calculate $39^2$. What number does he subtract?

 Aug 19, 2023
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39^2  =  (40 -1) (40 -1) =  40^2  - 2(40) + 1  =  1600 - 79

 

He subtracts 79

 

cool cool cool

 Aug 19, 2023

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