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a) `16x^2 + 24xy + a`

`= (4x)^2 + 2. 4x .3y + (3y)^2`

`= (4x+3y)^2 `

b) `a - 42xy + 49y^2`

`= (3x)^2 - 2.3x .7y + (7y)^2`

`= (3x-7y)^2`

c) `25x^2 + ..... + 81`

`= (5x)^2 + 2.5x . 9 + 9^2`

`= (5x+9)^2`

d) `64x^2- .... + 9`

`=  (8x)^2 - 2.8x.3 + 3^2`

`= (8x-3)^2`