Bài làm
3/1*4 + 3/4*7 + 3/7*10 + ... + 3/61*64
= 1 - 1/4 + 1/4 - 1/7 + 1/7 - 1/10 + ... + 1/61 - 1/64
= 1 - 1/64
= 64/64 - 1/64
= 63/64. ^.^
các bạn trả lời nhanh dùm minh nha ! mình đang cần gấp lắm đó
ĐẶT \(A=\frac{3}{1\times4}+\frac{3}{4\times7}+\frac{3}{7\times10}+......+\frac{3}{61\times64}\)
\(\Rightarrow A=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+.....+\frac{1}{61}-\frac{1}{64}\)
\(\Rightarrow A=1-\frac{1}{64}\)
\(\Rightarrow A=\frac{63}{64}\)
HAY \(\frac{3}{1\times4}+\frac{3}{4\times7}+\frac{3}{7\times10}+.....+\frac{3}{61\times64}=\frac{63}{64}\)
\(\frac{3}{1.4}+\frac{3}{4.7}+...+\frac{3}{61.64}\)
\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{61}-\frac{1}{64}\)
\(=1-\frac{1}{64}\)
\(=\frac{63}{64}\)
\(\frac{3}{1x4}+\frac{3}{4x7}+\frac{3}{7x10}+...+\frac{3}{61x64}\)
\(=\frac{3}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{61}-\frac{1}{64}\right)\)
\(=1.\left(1-\frac{1}{64}\right)\)
\(=\frac{63}{64}\)
