$\frac14a^2-3\mathrm{ab}+9b^2$

$4b^2-c^2$

$4x^2-12\mathrm{xy}+9y^2$

$\frac14a^2-\mathrm{ab}+b^2$

$a^2-b^2$

$49p^2-81q^2$

$a^2+16-8a$

$a^2+10a+25$

$5x^2+3xy+y^2+xy-x^2$

$\frac{36}{49}m^2-\frac3{14}mn+\frac1{64}n^2$

$\frac19u^2-\frac4{15}uv+\frac4{25}v^2$