Question

Câu 22 giải hộ em đi

Answer 1

\(\dfrac{2}{5\times7}+\dfrac{2}{7\times9}+...+\dfrac{2}{13\times15}\)

=\(\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{13}-\dfrac{1}{15}\)

=\(\dfrac{1}{5}-\dfrac{1}{15}\)

=\(\dfrac{2}{15}\)

Chọn A

Answer 2

Ta có: \(\dfrac{2}{5\cdot7}+\dfrac{2}{7\cdot9}+\dfrac{2}{9\cdot11}+...+\dfrac{2}{13\cdot15}\)

\(=\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{15}\)

\(=\dfrac{1}{5}-\dfrac{1}{15}=\dfrac{2}{15}\)

Answer 3

     \(\dfrac{2}{5\times7}+\dfrac{2}{7\times9}+\dfrac{2}{9\times11}+\dfrac{2}{11\times13}+\dfrac{2}{13\times15}\)

⇔ \(\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{15}\)

⇔  \(\dfrac{1}{5}-\dfrac{1}{15}\)

⇔  \(\dfrac{2}{15}\)

Câu này chọn đáp án  A

Answer 4

Chọn A

Answer 5

Ta có: =15−17+17−19+19−111+111−113+113−115