Question

tính nhanh

A=1/3+1/8+1/15+1/24+1/35+1/48+1/63+1/80

B=1/4+1/10+1/18+1/28+1?40+1/54+1/70

Answer 1

A=\(\frac{1}{3}\)+\(\frac{1}{8}\)+\(\frac{1}{15}\)+\(\frac{1}{24}\)+\(\frac{1}{35}\)+\(\frac{1}{48}\)+\(\frac{1}{63}\)+\(\frac{1}{80}\)

A=\(\frac{1}{2}\)(\(\frac{1}{1\cdot3}\)+\(\frac{1}{2\cdot4}\)+\(\frac{1}{3\cdot5}\)+\(\frac{1}{4.6}\)+\(\frac{1}{5.7}\)+\(\frac{1}{6.8}\)+\(\frac{1}{7.9}\)+\(\frac{2}{8.10}\))

A=\(\frac{1}{2}\)(1-1/3 +1/2-1/4 + 1/3 -1/5 +1/4-1/6 +1/5 - 1/7 +1/6 -1/8 +1/7 - 1/9 +1/8 - 1/10)

A= \(\frac{1}{2}\)(1 + 1/2 -1/9 -1/10)

A=\(\frac{29}{45}\)