Triển khai các biểu thức sau theo hằng đẳng thức

$13) (x-1).(x+1)=x^{2}-1$

$14) (x-5).(x+5)=x^{2}-5^{2}=x^{2}-25$

$15) (x-6).(6+x)=(x-6).(x+6)=x^{2}-6^{2}=x^{2}-36$

$16) (2x+1).(2x-1)=(2x)^{2}-1=4x^{2}-1$

$17) (x-2y).(2y+x)=(x-2y).(x+2y)=x^{2}-(2y)^{2}=x^{2}-4y^{2}$

$18) (5x-3y).(3y+5x)=(5x-3y).(5x+3y)=(5x)^{2}-(3y)^{2}=25x^{2}-9y^{2}$

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$NOTE: A^{2}-B^{2}=(A-B).(A+B)$

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$anhhtky$