có:2B+3=3+2B=3+2*3+2*3²+2*3^3+2*3^4+.......
=3²+2*3²+2*3^3+2*3^4+....+2*3^100=3^3+...
=......=3^101
3^x=3^101 <=> x=101
5 + 52 + 53 + ... + 596 (có 96 số, 96 chia hết cho 6)
= (5 + 52 + 53 + 54 + 55 + 56) + (57 + 58 + 59 + 510 + 511 + 512) + ... + (591 + 592 + 593 + 594 + 595 + 596)
= (5 + 54) + (52 + 55) + (53+ 56) + ... + (591 + 594) + (592 + 595) + (593+ 596)
= 5.(1 + 53) + 52.(1 + 53) + 53.(1 + 53) + ... + 591.(1 + 53) + 592.(1 + 53) + 593.(1 + 53)
= 5.126 + 52.126 + 53.126 + ... + 591.126 + 592.126 + 593.126
= 126.(5 + 52 + 53 + ... + 591 + 592 + 593) chia hết cho 126
=> đpcm
