a) `A=(5x+3y)^{2}-(3y-1)(1+3y)-(4-5x)^{2}-10x(3y+4)`
`=25x^{2}+30xy+9y^{2}-9y^{2}+1-16+40x-25x^{2}-30xy-40x`
`=(25x^{2}-25x^{2})+(30xy-30xy)+(9y^{2}-9y^{2})+(40x-40x)+(1-16)`
`=1-16`
`=-15` `(đpcm)`
b) `B=((x+5)^{2}+(x-5)^{2})/(x^{2}+25)`
`=(x^{2}+10x+25+x^{2}-10x+25)/(x^{2}+25)`
`=(2x^{2}+50)/(x^{2}+25)`
`=(2(x^{2}+25))/(x^{2}+25)`
`=2` `(đpcm)`
