\(\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+...+\dfrac{1}{9702}\\ =\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+...+\dfrac{1}{98\cdot99}\\ =\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{98}-\dfrac{1}{99}\\ =\dfrac{1}{3}-\dfrac{1}{99}\\ =\dfrac{32}{99}\)
Đề ra: Tính nhanh
1/12+1/20+1/30+.............+1/9702
Tính nhanh:
\(\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+...+\dfrac{1}{9702}\)
tìm x biết ; \(\frac{x-1}{12}+\frac{x-1}{20}+\frac{x-1}{30}+...+\frac{x-1}{9702}=\frac{32}{11}\)
1/18+1/20+...+1/9702
Tính
A= 2+6+12+20+30+......+9702+9900
Giúp mình vs nha
a,3/12*2+3/3*20+3/4*30+...+3/97*9702
b,1/1*6+1/6*11+...+1/491+496+1/496*501
c,1/1*2+1/2*3+1/3*4+...+1/199*200
d,2/1*2+2/2*3+2/3*4+...+2/2015*2016
e,1+1/3+1/6+...+1/495
Cho tổng A=1/2+5/6+11/12+19/20+...+9701/9702+9899/9900
Chứng tỏ A<99
N bằng 2/2*12+2/3*20+3/4*30+...+3/97*9702+3/98*9000
Giúp mk nha.
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{9702}+\frac{1}{9900}\)
chúc bạn làm tốt !
