Đáp án:
Giải thích các bước giải:
`3+3^3+3^5+3^7+.......+3^1991`
`=(3+3^3+3^5)+(3^7+3^9+3^11)+...+(3^1987+3^1989+3^1991)`
`=3(1+3^2+3^4)+3^7(1+3^2+3^4)+...+3^1987(1+3^2+3^4)`
`=3.91+3^7.91+...+3^1987.91`
`=91.(3+3^7+...+3^1987) \vdots91`
Mà `91\vdots13`
`=> 3+3^3+3^5+3^7+.......+3^1991\vdots13(đpcm)`