Tính tổng:
D=3-32-33-..........-3100
B=-2/15-2/35-2/63-..........-2/399
1.So sánh:
a, 2 mũ 6 và 6 mũ 2
b, 73+1 và 7 và 73 + 1
c, 1314 - 1313 và 1315 - 1314
d, 32+n và 23+n (n e N *)
2. Rút gọn mỗi biểu thức sau:
a) A= 1+3+32+33+.....+399+3100
b) B= 2100-299+298-297+....-23+22-2+1
tính :2/15+2/35+2/63+...+2/ 399 = ?
\(\frac{2}{15}+\frac{2}{35}+...+\frac{2}{399}\)
\(=\frac{2}{3.5}+\frac{2}{5.7}+....+\frac{2}{19.21}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}\)
\(=\frac{1}{3}-\frac{1}{21}\)
\(=\frac{2}{7}\)
HT
Tính: 2/15 + 2/35 + 2/63+.....+ 2/399
\(A=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\)
\(A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)
\(A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}\)
\(A=\frac{1}{3}-\frac{1}{21}=\frac{2}{7}\)
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
= 7/21 - 1/21
= 6/21 = 2/7
tính hợp lí:
2/15+2/35+2/63+......+2/399
\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+..+\frac{2}{399}\)
\(=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\)
\(=\frac{1}{3}-\frac{1}{21}=\frac{7}{21}-\frac{1}{21}=\frac{6}{21}=\frac{2}{7}\)
Ủng hộ mk nha !!! ^_^
B = 2/15 + 2/35 + 2/63 +...+ 2/399
\(B=\frac{2}{3.5}+\frac{2}{5.7}+......+\frac{2}{29.31}\)
\(B=2.\left(\frac{1}{3.5}+\frac{1}{5.7}+....+\frac{1}{29.31}\right)\)
\(B=2.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+.....+\frac{1}{29}-\frac{1}{31}\right)\)
\(B=2.\left(\frac{1}{3}-\frac{1}{31}\right)\)
\(B=2.\frac{28}{93}\)
\(B=\frac{56}{93}\)
k cho mình nha chúc bạn học tốt
Bài 1: tính
A=2/15 +2/35 + 2/63 +.......+ 2/399
A=1/3+1/6+1/10+1/15+...+1/66
B= 2/15+2/35+2/63+..+2/399
\(B=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\)
\(B=\frac{2}{3×5}+\frac{2}{5×7}+\frac{2}{7×9}+...+\frac{2}{19×21}\)
\(B=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{19}-\frac{1}{21}\)
\(B=\frac{1}{3}-\frac{1}{21}\)
\(B=\frac{2}{7}\)
A=\(\frac{1}{3}\)+\(\frac{1}{6}\)+\(\frac{1}{10}\)+\(\frac{1}{15}\)+...+\(\frac{1}{66}\)
A=\(\frac{1}{1\cdot3}\) +\(\frac{1}{2\cdot3}\) +\(\frac{1}{2\cdot5}\)+...+\(\frac{1}{6\cdot11}\)
A=\(\frac{1}{1}-\frac{1}{3}+\frac{1}{2}-\frac{1}{3}+\frac{1}{2}-\frac{1}{5}+...+\frac{1}{6}-\frac{1}{11}\)
A=\(\frac{1}{1}-\frac{1}{11}\)
=>A=\(\frac{10}{11}\)
B=\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\)
2B=\(\frac{1}{3\cdot5}+\frac{1}{5\cdot7}+\frac{1}{7\cdot9}+...+\frac{1}{19\cdot21}\)
2B=\(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\)
2B=\(\frac{1}{3}-\frac{1}{21}\)
2B=\(\frac{2}{7}\)
B=\(\frac{2}{7}:2\)
=>B=\(\frac{1}{7}\)
a = 2/15 + 2/35 + 2/63 + ... + 2/399 = ?
sai đề rồi bạn ơi!bạn hãy kiểm tra lại bài đi nha
A=\(\dfrac{2}{15}\)+\(\dfrac{2}{35}\)+\(\dfrac{2}{63}\)+...+\(\dfrac{2}{399}\)
`A =2/15 +2/35 +2/63 +... +2/339`
`= 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 = 7/21 -1/21`
`=6/21 = 2/7`
sai rồi,2/7 mới đúng,bài này không cần nhân 2