hép mik chỉ nhận hôm nay đến thứ 7 thoi giải chi tiết nha ai đúng mik tích cho
2.
\(a.\dfrac{2}{5\times7}+\dfrac{2}{7\times9}+\dfrac{2}{9\times11}+...+\dfrac{2}{2023\times2025}\\ =\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+...+\dfrac{1}{2023}-\dfrac{1}{2025}\\ =\dfrac{1}{5}-\dfrac{1}{2025}\\ =\dfrac{404}{2025}\\ b.\dfrac{3}{1\times4}+\dfrac{3}{4\times7}+\dfrac{3}{7\times10}+\dfrac{3}{10\times13}\\ =1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{13}\\ =1-\dfrac{1}{13}\\ =\dfrac{12}{13}\)
1
`(x-5)/12 = 45/60`
`=> x-5 = 9`
`=> x = 14`
Vậy `x = 14`
`b) ( x + 1/(5 .6)) + (x + 1/(6.7)) + (x + 1/(7.8)) + (x + 1/(8.9)) = 139/45`
`=> x + 1(5.6) + x + 1/(6.7) + x + 1/(7.8) + x + 1/(8.9) = 139/45`
`=> 4x + (1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9) = 139/45`
`=> 4x + (1/5 - 1/9) = 139/45`
`=> 4x + 4/45 = 139/45`
`=> 4x = 3`
`=> x = 3/4`
Vậy `x = 3/4`
