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Question

Tính A=13x15+15x17+17x19+...+99x101

Nguyễn Đức Trí · Accepted Answer

$A=13.15+15.17+17.19+...+99.101$

$\Rightarrow6A=13.15.6+15.17.6+17.19.6+...+99.101.6$

$\Rightarrow6A=13.15.\left(17-11\right)+15.17.\left(19-13\right)+17.19.\left(21-15\right)+...+99.101.\left(103-97\right)$

$\Rightarrow6A=\left(13.15.17-11.13.15\right)+\left(15.17.19-13.15.17\right)+\left(17.19.21-15.17.19\right)+...+\left(99.101.103-97.99.101\right)$

$\Rightarrow6A=99.101.103-11.13.15$

$\Rightarrow6A=1027752$

$\Rightarrow A=171292$Câu trả lời: $A=13.15+15.17+17.19+...+99.101$

$\Rightarrow6A=13.15.6+15.17.6+17.19.6+...+99.101.6$

$\Rightarrow6A=13.15.\left(17-11\right)+15.17.\left(19-13\right)+17.19.\left(21-15\right)+...+99.101.\left(103-97\right)$

$\Rightarrow6A=\left(13.15.17-11.13.15\right)+\left(15.17.19-13.15.17\right)+\left(17.19.21-15.17.19\right)+...+\left(99.101.103-97.99.101\right)$

$\Rightarrow6A=99.101.103-11.13.15$

$\Rightarrow6A=1027752$

$\Rightarrow A=171292$