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`a)`

`501^2`

`=` `(500+1)^2`

`=` `500^2+2.500.1+1^2`

`=` `250000+1000+1`

`=` `251001`

`b)`

`72^2+28^2+72.56`

`=` `72^2+72.56+28^2`

`=` `72^2+2.72.28+28^2`

`=` `(72+28)^2`

`=` `100^2`

`=` `10000`

`c)`

`98^2`

`=` `(100-2)^2`

`=` `100^2-2.100.2+2^2`

`=` `10000-400+4`

`=` `9604`