Tinh : A = 2+22+222+2222+222..2 ( so cuoi cung co 40 chu so 2 )
Cho tong S=\(\frac{2}{3.7}+\frac{2}{7.11}+\frac{2}{11.15}+.....\)co 100 so hang
a] tim so hang cuoi cung cua day
b]tinh S
tinh tong: D=2+22+222+...+22...2 (10 co so)
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
Tinh
D = 1+4+9+16+..........+100
Quy tac :
Buoc 1: So so hang = (so cuoi - so dau) ÷ khoang cach + 1
Buoc 2 : tong = (so cuoi + so dau ) × so so hang ÷ 2
co tat ca bao nhieu so co 4 chu so trong do 2 chu so dau giong nhau va 2 chu so cuoi giong nhau ( biet rang trong moi so do co chu so hang tram khac chu so hang chuc)
1. tim so tu nhien co 3 chu so,biet so do gap 20 lan tong cac chu so cua no
2. tim so tu nhien co 2 chu so, biet rang neu lay so do chia cho hieu cua chu so hang chuc va hang don vi, thi ta duoc thuong la 28 du 1
3. tim 1 so tu nhien co ba chu so , biet rang neu chuyen chu so cuoi len truoc chu so dau ta duoc so moi hon so da cho 765 don vi
tinh gia tri bieu thuc
U=3+33+333+....+333333...3(co 100 chu so 3 o so hang cuoi)
tim tat cac so tu nhien co ba chu so thoan man cac tinh chat sau : co tong cac chu so bang 9 chu so hang chuc lom hon chu so hang don vi la 2 don vi va neu ta doi chu so hang tram cho hang don vi thi duoc so moi lon hon so ban dau la 198 don vi
