Đáp án: $0$

Giải thích các bước giải:

$(1.2+2.3+3.4+...+19.20)(\sqrt{169}-2)(\sqrt{169}-3)(\sqrt{169}-4)...(\sqrt{169}-20)$

$=(1.2+2.3+3.4+...+19.20)(\sqrt{169}-2)(\sqrt{169}-3)(\sqrt{169}-4)...(\sqrt{169}-13)...(\sqrt{169}-20)$

$=(1.2+2.3+3.4+...+19.20)(\sqrt{169}-2)(\sqrt{169}-3)(\sqrt{169}-4)...(\sqrt{13^2}-13)...(\sqrt{169}-20)$

$=(1.2+2.3+3.4+...+19.20)(\sqrt{169}-2)(\sqrt{169}-3)(\sqrt{169}-4)...(13-13)...(\sqrt{169}-20)$

$=(1.2+2.3+3.4+...+19.20)(\sqrt{169}-2)(\sqrt{169}-3)(\sqrt{169}-4)...0...(\sqrt{169}-20)$

$=0$