Question

Tính B=1+1/2(1+2)+1/3.(1+2+3)+...+1/20.(1+2..+20)

Answer 1

\(B=1+\frac{1}{2}\left(1+2\right)+\frac{1}{3}\left(1+2+3\right)+...+\frac{1}{20}\left(1+2+3+...+20\right)\)

\(=1+\frac{1}{2}2.3:2+\frac{1}{3}.3.4:2+...+\frac{1}{20}.20.21:2=\frac{2}{2}+\frac{3}{2}+...+\frac{21}{2}\)

\(=\frac{2+3+4+...+21}{2}=\frac{230}{2}=115\)

Answer 2

B = 1+1/2×(2×3/2)+1/3×(3×4/2)+1/4×(4×5/2)+...+1/20×(20×21/2)=1+3/2+4/2+...+21/2=1/2×(2+3+4+...+21=1/2×(2+3+4+...+21)=1/2×(21×22/2-1)=115

Answer 3

b=1+1+2/2+1+2+3/3+1/1+2+3+4/4+..+1+2+3+...+20/20

b=1+1/2+2/2+1/3+2/3+3/3+...+1/20+2/20+3/20+...+19/20

b=1+2+4+5+7+8+10+11+13+14+16+17+19+20

b=147

Answer 4

	\(\frac{23}{2}=\frac{2}{3\hept{\begin{cases}\\\end{cases}}}\)