Đáp án:
`A= 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 `
`A = 1/(1.2) + 1/(2 . 3) + 1/(3.4) + 1/(4.5) + 1/(5.6) + 1/(6.7) + 1/(7.8)`
`A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8`
`A = 1 + [(-1/2) + 1/2] + [(-1/3) + 1/3] + [(-1/4) + 1/4] + [(-1/5) + 1/5] + [(-1/6) + 1/6] + [(-1/7) + 1/7] - 1/8`
`A = 1 + 0 + 0 + 0 + 0 + 0 + 0 - 1/8`
`A = 1 - 1/8`
`A = 8/8 - 1/8 `
`A = 7/8`
`B = 3/2+5/6+7/12+-9/20+11/30-13/42+15/56`
Sửa lại đề:
`B = 3/2 - 5/6 + 7/12 - 9/20 + 11/30 - 13/42 + 15/56`
`B = 3/(1.2) - 5/(2.3) + 7/(3.4) - 9/(4 . 5) + 11/(5.6) - 13/(6.7) + 15/(7.8)`
`B = 3 - 3/2 -5/2 -(- 5/3) + 7/3 - 7/4 - 9/4 -(- 9/5) + 11/5 - 11/6 - 13/6 -(- 13/7) + 15/7 - 15/8`
`B = 3 - 3/2 -5/2 + 5/3 + 7/3 - 7/4 - 9/4 + 9/5 + 11/5 - 11/6 - 13/6 + 13/7 + 15/7 - 15/8`
`B = 3 + (-3/2 - 5/2) + (5/3 + 7/3) + (-7/4 - 9/4) + (9/5 + 11/5) + (-11/6 - 13/6) +( 13/7 + 15/7) - 15/8`
`B = 3 + (-4) + 4 + (-4) + 4 + (-4) + 4 - 15/8`
`B = 3 + [(-4) + 4] +[ (-4) + 4] +[ (-4) + 4] - 15/8`
`B = 3 + 0 + 0 + 0 - 15/8`
`B = 3 - 15/8`
`B = 24/8 - 15/8`
`B = 9/8`
