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1 + 1/3 + 1/6 + 1/10 + .......... + 1/x.(x+1):2 =1 + 1991/1993
1/2.(1 + 1/3 + 1/6 + 1/10+........+ 1/x.(x+1):2=3984/3986
1/2 + 1/6 +1/12 + .......... +1/x.(x+1)=3984/3986
1/1.2 + 1/2.3 + 1/3.4 +..........+.1/x.(x+1)=3984/3986
2-1/1.2 + 3-2/2.3 + 4-3/3.4 +..........+ x + 1 - x/x.(x+1)
1-1/2+1/2-1/3+1/3-1/4+..........+1/x -1/x+1 =3984/3986
1-1/x+1=3984/3986
1/x+1=1-3984/3986
1/x+1=2/3986=1/1993
x+1=1993
x =1993-1
x =1992
Bn nên viết cái đề rõ ra nha !
Bài 3 :
\(\left[3\left(x+2\right):7\right].4=120\)
\(3\left(x+2\right):7=120:4\)
\(3\left(x+2\right):7=30\)
\(3\left(x+2\right)=30.7\)
\(3\left(x+2\right)=210\)
\(x+2=210:3\)
\(x+2=70\)
\(x=70:2\)
\(x=35\)
3b.
\(1313\div\left(2x\div5\right)=13\)
\(2x\div5=1313\div13\)
\(2x\div5=101\)
\(2x=101.5\)
\(2x=505\)
\(x=252,5\)
3a,
\(\left[3\left(x+2\right):7\right].4=120\)
\(3\left(x+2\right):7=120:4\)
\(3\left(x+2\right):7=30\)
\(3\left(x+2\right)=30.7\)
\(3\left(x+2\right)=210\)
\(x+2=210:3\)
\(x+2=70\)
\(x=68\)
b, 1313 : ( 2x : 5 ) = 13
2x : 5 = 1313 : 13
2x : 5 = 101
2x = 101.5
2x = 505
x = 252,5
a. 100 - 7 ( x - 5 ) = 58
<=> 7 ( x - 5 ) = 100 - 58
<=> 7 ( x - 5 ) = 42
<=> x - 5 = 42 : 7
<=> x - 5 = 6
<=> x = 6 + 5
<=> x = 11
Tương tự tiếp.
a;100-7(x-5)=58
=>7(x-5)=100-58=42
=>x-5=42:7=6
=>x=6+5=11
b;12(x-1):3=72
=>12(x-1)=72.3=216
=>x-1=216:12=18
=>x=18+1=19
c;12-4(x-1)=4
=>4(x-1)=12-4=8
=>x-1=8:4=2
=>x=2+1=3
d;32-12x=8
=>12x=32-8=24
=>x=24:12=2
nho h do nhe viet moi tay lam day biet ko
-3/2+-1/-12=-17/12
-12/24+-1/-2=0
k mk nha