Dạ thưa Vũ a = 99

\(\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{a.\left(a+1\right)}=\frac{49}{100}\)

\(\Rightarrow\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{a}-\frac{1}{a+1}=\frac{49}{100}\)

\(\Rightarrow\frac{1}{2}-\frac{1}{a+1}=\frac{49}{100}\)

\(\Rightarrow\frac{1}{a+1}=\frac{1}{100}\)

\(\Rightarrow a+1=100\)

\(\Rightarrow a=99\)

=>1/2-1/3+1/3-1/4+...+1/a-1/a+1=49/100

=>1/2-1/a+1=49/100

=>1/a+1=49/100+1/2

=>1/a+1=99/100

=>\(\frac{99}{\left(a+1\right).99}=\frac{99}{100}\)

=>(a+1).99=100

=>a+1=100/99

=>a=100/99-1

=>a=1/99

Sheein chuyển vế sai rồi dẫn đến tìm x sai

\(\frac{1}{2\times3}+\frac{1}{3\times4}+............+\frac{1}{a\times\left(a+1\right)}=\frac{49}{100}\)

\(\Rightarrow\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+..............+\frac{1}{a}-\frac{1}{a+1}=\frac{49}{100}\)

\(\Rightarrow\frac{1}{2}-\frac{1}{a+1}=\frac{49}{100}\)

\(\Rightarrow\frac{1}{a+1}=\frac{1}{2}-\frac{49}{100}\)

\(\Rightarrow\frac{1}{a+1}=\frac{1}{100}\)

\(\Rightarrow a+1=100\)

\(\Rightarrow a=99\)

Đáp số là a = 99 nha còn cách làm thì  Nguyễn Hung Phat đã làm rồi nha

T ik nha bạn =))

Chúc bạn học tốt nhé !!!

1.

a.

\(\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{5\times6}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}=1-\frac{1}{6}=\frac{5}{6}\)

b.

Tích có 100 thừa số 

=> n = 100

\(\left(100-1\right)\times\left(100-2\right)\times\left(100-3\right)\times...\times\left(100-99\right)\times\left(100-100\right)\)

\(=\left(100-1\right)\times\left(100-2\right)\times\left(100-3\right)\times...\times\left(100-99\right)\times0\)

\(=0\)

2.

a.

\(135\times789789-789\times135135=1001\times\left(135\times789-789\times135\right)=1001\times0=0\)

b.

\(\left(28\times9696-96\times2828\right)\div\left(1\times2\times3\times...\times2015\times2016\right)\)

\(=\left[101\times\left(28\times96-96\times28\right)\right]\div\left(1\times2\times3\times...\times2015\times2016\right)\)

\(=\left(101\times0\right)\div\left(1\times2\times3\times...\times2015\times2016\right)\)

\(=0\div\left(1\times2\times3\times...\times2015\times2016\right)\)

\(=0\)

3.

a.

\(\left[\left(x+32\right)-17\right]\times2=42\)

\(\left(x+32\right)-17=\frac{42}{2}\)

\(\left(x+32\right)-17=21\)

\(x+32=21+17\)

\(x+32=38\)

\(x=38-32\)

\(x=6\)

b.

\(125+\left(145-x\right)=175\)

\(145-x=175-125\)

\(145-x=50\)

\(x=145-50\)

\(x=95\)

A=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6

A=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6

A=1-1/6

A=5/6

Vậy: A=5/6

B=(100-1).(100-2).(100-3). ... .(100-n)

B=99.98.97. ... .0

B=0

Vậy B=0

              Chắc Vậy

ai làm nhanh mik cho

mik cho

haizzz đáng tiếc tôi muốn ns là:  ko bao f và đừng mong chờ OK