Question

B =1/1.4 + 1/4.7+1/7.10+...+1/397.400 giup mik vs

nãy quên khi B=

giúp mik với

Answer 1

\(\dfrac{1}{1.4}+\dfrac{1}{4.7}+\dfrac{1}{7.10}+...+\dfrac{1}{397.400}\)

\(=\dfrac{1}{3}.\left(\dfrac{3}{1.4}+\dfrac{3}{4.7}+\dfrac{3}{7.10}+...+\dfrac{3}{397.400}\right)\)

\(=\dfrac{1}{3}.\left(\dfrac{4-1}{1.4}+\dfrac{7-4}{4.7}+\dfrac{10-7}{7.10}+...+\dfrac{400-397}{397.400}\right)\)

\(=\dfrac{1}{3}.\left(1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{397}-\dfrac{1}{400}\right)\)

\(=\dfrac{1}{3}.\left(1-\dfrac{1}{400}\right)\)

\(=\dfrac{1}{3}.\dfrac{399}{400}\)

\(=\dfrac{133}{400}\)

Answer 2

cho hoi ne gio B= thi tinh sao vay ban