Ta có 11..11 ( n số ) = (10^n-1):9
= 2 ( 1 + 11 + 111 + ................. + 11...1 )
= 2 ( 10^1 + 10^2 + 10^ 3 + ........ + 10^ 50 - 50 ) :9
10 ^ 1 + 10^ 2 + 10^ 3 + ................ + 10^ 50 -50 = (10^51 - 1 ) : 9 -50
Biểu thức = 2 ( 10^51 - 451 ) : 81
tinh tong: 2+22+...+222...2 o so hang cuoi cung co n chu so 2
Tinh : A = 2+22+222+2222+222..2 ( so cuoi cung co 40 chu so 2 )
2+22+222+...+22...2(co 50 chu so 2)
2+22+222+...+22...2(co 50chu so 2)
cho S = 7 + 10 + 13 + ...97 + 100
a. tong tren co bao nhieu so hang
b. tim so hang thu 22
c. tinh S
bai 1
cho tong : S = 5 + 8 + 11 +14 = ....
a) tim so hang thu 100 cua tong
b) tinh tong 100 so hang dau tien
bai 2
cho tong : S = 7 + 12 + 17 + 22 + .......
a) tim so hang thu 50 cua tong
b) tinh tong 50 so hang dau tien
bai 3
tinh tong cua tat ca cac so hang tu nhien x , biet x la so tu nhien co 2 chu so va 12 < x <91
ban nao lam ho minh 3 bai tren voi ma ban nao nao lam ra dau tien minh se tich ngay va lun
Cho tong S =7+12+17+22+...
a,Tim so hang thu 50
b,Tinh tong cua 50 so hang dau tien
tinh nhanh tong
q= 2+4+6+.......+22
Tính 2+22+222+2222+....+(222....22); trong ngoặc có 50 số 2.
