\(A=1.2+2.3+...+25.26\)
\(\Rightarrow3A=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+25.26.\left(27-24\right)\)
\(=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+25.26.27-24.25.26\)
\(\Rightarrow3A=25.26.27\)
\(\Rightarrow A=25.26.9\)
S=1x2+2x3+3x4+.....+99x100
Tính tổng sau :
1 /1x2 + 1/2x3 + 1/3x4 + ...+ 1/999x1000 + 1
Tính: S= 1x2+2x3+3x4+... +38x39+39x40
1. Tính nhanh:
a. \(\dfrac{1}{3}+\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+\dfrac{5}{12}+\dfrac{19}{30}\)
b. \(\dfrac{1}{1x2}+\dfrac{1}{2x3}+\dfrac{1}{3x4}+...+\dfrac{1}{1998x1999}+\dfrac{1}{1999x2000}\)
Đây là toán lớp 5 nhé!
tính:
1/1x2+1/1x3+1/3x4+..............1/n+(n+1)
tính:
1/1x2+1/1x3+1/3x4+..............1/n+(n+1)=................................................
Tính nhanh:
A,(\(\dfrac{8}{15}\)+\(\dfrac{14}{23}\))-(\(\dfrac{5}{15}\)-\(\dfrac{9}{23}\))
B,\(\dfrac{1}{1x2}\)+\(\dfrac{1}{2x3}\)+\(\dfrac{1}{3x4}\)+\(\dfrac{1}{4x5}\)+\(\dfrac{1}{5x6}\)
Tính: 1x2 + 3x4 + 5x6 + 7x8 + 9x10 + 11x12 + 13x14 + 15x16
Chú ý: ko được tính ra rồi cộng vào
