lets consider the number to be 2x.

to find out the square of 2x,simply:

1)always start with the number 400

2)then multiply 40 with your one's digit(we have taken it as 'x')

now, add both..400+40x

3)add x^2 to 400+40x

so, the answer of 2x^2 is 400+40x+x^2

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