a) S=30+32+34+...+32002
\(\Rightarrow\)9S=32+34+36+...+32004
\(\Rightarrow\)9S-S=(32+34+36+...+32004)-(1+32+34+...+32002)
8S=32004-1
\(\Rightarrow S=\frac{3^{2004}-1}{8}\)
b) Ta có : S=1+32+34+...+32002
=(1+32+34)+(36+38+310)+...+(31998+32000+32002)
=1(1+32+34)+36(1+32+34)+...+31998(1+32+34)
=1.91+36.91+...+31998.91
Mà 91\(⋮\)7 nên 1.91+36.91+...+31998.91\(⋮\)7
\(\Rightarrow S⋮7\)(đpcm)
a) S=30+32+34+36+.....+32002
=>32S=32+34+36+.....+32002+32004
=>9S-S=(32+34+36+.....+32002+32004)-(30+32+34+36+.....+32002)
=>8S=32004 - 1
=>S=(32004 - 1) / 8
b) S= 30+32+34+36+.....+32002
S=(30+32+34)+(36+38+310)+.....+(31998+32000+32002)
S=91+36(30+32+34)+.....+31998(30+32+34)
S=91.1+36.91+....+31998.91
S=91(1+36+....+31998) chia hết cho 7
=>S chia hết cho 7
Câu a mk ko chắc làm đúng ko nữa
\(S=3^0+3^2+3^4+3^6+....+3^{2002}\)
\(S=3^2.\left(3^0+3^2+3^4+3^6+...+3^{2002}\right)\)
\(2S=3^2+3^4+3^6+3^8+...+3^{2004}\)
\(S=\left(3^2+3^4+3^6+3^8+..+3^{2004}\right)-\left(3^0+3^2+3^4+3^6+....+3^{2002}\right)\)
triệt tiêu ta còn lại:
\(2S=\left(3^{2004}-3^0\right)\)
=>\(S=\left(3^{2004}-3^0\right):2\)
Cho mk hỏi bn khánh linh nha nếu bn nhân 2 với S thì vd như 30 x 2 mà bằng đc 32 à bn
