`a)87^2-13^2`
`=(87-13)(87+13)`
`=74.100=7400`
`b)53^2-2.53.3+3^2`
`=(53-3)^2`
`=50^2=2500`
`c)99^2`
`=(100-1)^2`
`=100^2-2.100+1`
`=10000-100+1`
`=9900+1`
`=9901`
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