\(\frac{1}{1}+2+\frac{1}{1}+2+3+\frac{1}{1}+2+3+4+\frac{1}{1}+2+3+4+5\)
\(=1+2+1+2+3+1+2+3+4+1+2+3+4+5\)
\(=\left(1\times4\right)+\left(2\times4\right)+\left(3\times3\right)+\left(4\times2\right)+5\)
\(=4+8+9+8+5\)
\(=4+5+9+8\times2\)
\(=4+5+9+16\)
\(=\left(4+16\right)+14\)
\(=20+14=34\)
k cho tớ nha bn
\(\frac{1}{1+2}+\frac{1}{1+2+3}+\frac{1}{1+2+3+4}+\frac{1}{1+2+3+4+5}\)
\(=\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}\)
\(=\frac{2}{3}\)
bạn sakura trả lời đúng nhưng không ai k nên tui k cho
