Tính A= 1/15 + 1/35 + 1/63+ 1/99 + ... + 1/999
A= 1/15+1/35+1/63+1/99+.............+1/999
\(A=\frac{1}{3.5}+\frac{1}{5.7}+....+\frac{1}{99.101}\)
\(A=\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-...-\frac{1}{101}\right)=\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{101}\right)\)
\(A=\frac{49}{303}\)
ta có:
A=\(\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}+....+\frac{1}{999}\)
A=\(\frac{1}{3\text{x}5}+\frac{1}{5\text{x}7}+\frac{1}{7\text{x}9}+\frac{1}{9\text{x}11}+...+\frac{1}{?\text{x}?}\)
A=
1/15+1/35+1/63+1/99+.............+1/999
Thử các mẫu số ta thấy
Các mẫu số có dạng x(x + 2)
Số 999 khác dạng x(x + 2)
Bạn xem lại đề
A = 1/15 + 1/35 + 1/ 63 + 1/99 + ...+ 1/9999
A = 1/(3x5) + 1/(5x7) + 1/(7x9) + 1/(9x11) + ... + 1/(99 x 101)
Ax2 = 2/(3x5) + 2/(5x7) + 2/(7x9) + 2/(9x11) + ... + 2/(99 x 101)
Ax2 = 1/3 – 1/5 + 1/5 – 1/7 + 1/7 – 1/9 + 1/9 – 1/11 + ...+ 1/99 – 1/101
Ax2 = 1/3 – 1/101 = 98/303
A = 98/303 : 2
A = 49/303
1.tính nhanh giá trị biểu thức biết :A=1+3/15+3/35+3/63+3/99+3/143
2.cho m =999....999;n=777...77(100 chữ số 9;100 chữ số 7),tính tổng các chữ số của m*n
Tính A= 1/15+ 1/35 + 1/63 +1/99 +...+ 1/9999
\(A=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{99.101}\)
\(A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+....+\frac{1}{99}-\frac{1}{101}\)
\(A=\frac{1}{3}-\frac{1}{101}\)
\(A=\frac{98}{303}\)
A = \(\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}+...+\frac{1}{9999}\)\(=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+...+\frac{1}{99.101}\)
= \(\left(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+...+\frac{2}{99.101}\right):2\)\(=\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{99}-\frac{1}{101}\right):2\)
= \(\left(\frac{1}{3}-\frac{1}{101}\right):2=\frac{101-3}{303}:2=\frac{98}{303}:2=\frac{49}{303}\)
Dấu chấm trong bài là dấu nhân nha !
Đặt \(A=\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+..+\frac{1}{9999}\)
\(A=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{99.101}\)
\(2A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{99.101}\)
\(2A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{99}-\frac{1}{101}\)
\(2A=\frac{1}{3}-\frac{1}{101}\)
\(A=\frac{98}{303}:2\)
\(A=\frac{49}{303}\)
Ủng hộ mk nha !!! ^_^
Tính nhanh : A = 1/15+1/35+1/63+1/99+.....+1/9999
Tính A=1-15 +1-35 +1-63 +1-99 +....+1-9999
Tính nhanh: A= 1/15+1/35+1/63+1/99+... +1/9999
\(\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+...+\frac{1}{9999}\)
\(\Rightarrow\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{99.101}\)
\(\Rightarrow\frac{1}{2}\left(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{99.101}\right)\)
\(\Rightarrow\frac{1}{2}\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{99}-\frac{1}{101}\right)\)
\(\Rightarrow\frac{1}{2}\left(\frac{1}{3}-\frac{1}{101}\right)\)
\(\Rightarrow\frac{1}{2}.\frac{98}{303}\)
\(\Rightarrow\frac{49}{303}\)
1/15 + 1/35 + 1/63 + 1/99 + ... + 1/9999 =
= 1/(3x5) + 1/(5x7) + 1/(7x9) + ... + 1/(99x101)
= (1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ...+ 1/99 - 1/101) : 2
= (1/3 - 1/101) : 2
= 98/303 : 2
= 49/303
Tính nhanh A=1/15 + 1/35+ 1/63 + 1/99 +... +1/9999
tinh nhanh A=1/15+1/35+1/63+1/99...+1/9999
A=
49/303,xin lỗi bạn mk làm biếng viết lời giải nếu cần nói mk nha
\(A=\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+........+\frac{1}{9999}\)
\(2A=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+.......+\frac{2}{9999}\)
\(2A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+......+\frac{2}{99.101}\)
\(2A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+.......+\frac{1}{99}-\frac{1}{101}\)
\(2A=\frac{1}{3}-\frac{1}{101}\)
\(2A=\frac{98}{303}\)
\(A=\frac{98}{303}:2=\frac{98}{303}.\frac{1}{2}=\frac{49}{303}\)
tính giá trị
a=1/15 +1/35+1/63+1/99