Toán 1. tính tổng: E=2/15+2/35+2/63+…….+2/399 23/11/2021 By Lyla 1. tính tổng: E=2/15+2/35+2/63+…….+2/399
Đáp án: \(E=\dfrac{2}{15}+\dfrac{2}{35}+\dfrac{2}{63}+…+\dfrac{2}{399}\) \(E=\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+…+\dfrac{2}{19.21}\) \(E=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+…+\dfrac{1}{19}-\dfrac{1}{21}\) \(E=\dfrac{1}{3}-\dfrac{1}{21}\) \(E=\dfrac{2}{7}\) Giải thích các bước giải: Trả lời
Đáp án: 2/15 + 2/35 + 2/63 + … +2/399 = 2/3.5 + 2/5.7 + 2/7.9 + … + 2/ 19.21 = 1/3 – 1/5 + 1/5 – 1/7 + 1/7 – 1/9 … 1/19 – 1/21 = 1/3 – 1/21 = 1/3 – 1/21 = 7/21 – 1/21 = 6/21 = 2/7 Trả lời
Đáp án:
\(E=\dfrac{2}{15}+\dfrac{2}{35}+\dfrac{2}{63}+…+\dfrac{2}{399}\)
\(E=\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+…+\dfrac{2}{19.21}\)
\(E=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+…+\dfrac{1}{19}-\dfrac{1}{21}\)
\(E=\dfrac{1}{3}-\dfrac{1}{21}\)
\(E=\dfrac{2}{7}\)
Giải thích các bước giải:
Đáp án:
2/15 + 2/35 + 2/63 + … +2/399
= 2/3.5 + 2/5.7 + 2/7.9 + … + 2/ 19.21
= 1/3 – 1/5 + 1/5 – 1/7 + 1/7 – 1/9 … 1/19 – 1/21
= 1/3 – 1/21
= 1/3 – 1/21
= 7/21 – 1/21
= 6/21
= 2/7