b/
`P =1 +3+3^2 +3^3+ ...+3^101`
`=> P =(1+ 3 +3^2) +(3^3+3^4+3^5) + ... + (3^96 + 3^97 +3^98 ) + (3^99 +3^100 +3^101)`
`=> P = 13 + 3^3(1 +3+3^2) +... +3^96(1 +3+3^2) + 3^99(1 +3+3^2)`
`=> P =13 +3^3 . 13 + ... +3^96 . 13 + 3^99 .13`
`=> P =13(1 +3^3 +... +3^96 + 3^99) \vdots 13` ( ĐPCM )