P = 1+2+22+23+24+25+26+27+28
P = (1+2+22)+(23+24+25)+(26+27+28)
P = 7+23.(1+2+22)+26.(1+2+22)
P = 7+23.7+26.7
P = 7.(23+26) chia hết cho 7
Vậy : 1+2+22+23+24+25+26+27+28 chia hết cho 7.
P = 1+2+22+23+24+25+26+27+28
= (1+2+22)+(23+24+25)+(26+27+28)
= 7 +23(1+2+22)+26(1+2+22)
= 7 + 23+7 +26+7
⇒p⋮7
P = 1+2+22+23+24+25+26+27+28
P = (1+2+22)+(23+24+25)+(26+27+28)
P = 7+23.(1+2+22)+26.(1+2+22)
P = 7+23.7+26.7
P = 7.(23+26) chia hết cho 7
Vậy : 1+2+22+23+24+25+26+27+28 chia hết cho 7.