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K

kieuquocdat

Dễ ợt: Đặt S=3+3^2+3^3+...3^120
\Leftrightarrow S=(3+...+3^4)+(3^5+...+3^8)+(3^9+...+3^12)+...+(3^117+...+3^120)
\Leftrightarrow S=1*(3+...+3^4)+3^4*(3+...+3^4)+3^8*(3+...+3^4)+...+ 3^116*(3+...+3^4)
\Leftrightarrow S=(1+3^4+3^8+...+3^116)*(3+...+3^4)
\Leftrightarrow S=(1+3^4+3^8+...+3^116)*120
\Rightarrow S chia hết cho 120 (đpcm).
 
T

tung2060

Đặt S=3+3^2+3^3+...3^120
S=(3+...+3^4)+(3^5+...+3^8)+(3^9+...+3^12)+...+(3^ 117+...+3^120)
S=1*(3+...+3^4)+3^4*(3+...+3^4)+3^8*(3+...+3^4)+.. .+ 3^116*(3+...+3^4)
S=(1+3^4+3^8+...+3^116)*(3+...+3^4)
S=(1+3^4+3^8+...+3^116)*120
Vay S chia hết cho 120 (đpcm).
 
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T

tung2060

Bai nay dung ko.............................................................................Con bai kho vao giai di
Neu giam chieu dai HCN 10% thi tang chieu rong bao nhieu % de S ko doi
 
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