1:
$\frac{3}{3x5}$ +$\frac{3}{5x7}$ +$\frac{3}{7x9}$ +......+$\frac{3}{97x99}$
A=$\frac{3}{3x5}$ +$\frac{3}{5x7}$ +$\frac{3}{7x9}$ +......+$\frac{3}{97x99}$
Ax$\frac{2}{3}$=($\frac{3}{3x5}$ +$\frac{3}{5x7}$ +$\frac{3}{7x9}$ +......+$\frac{3}{97x99}$ )x$\frac{2}{3}$
Ax$\frac{2}{3}$=$\frac{2}{3x5}$ +$\frac{2}{5x7}$ +$\frac{2}{7x9}$ +......+$\frac{2}{97x99}$
Ax$\frac{2}{3}$=2/3-2/5+2/5-2/7+.....+2/97-2/99
Ax$\frac{2}{3}$=2/3-2/99
Ax$\frac{2}{3}$=20/99
A=20/99:2/3
A=10/33
2:
(x+1)+(x+2)+(x+3)+....+(x+9)=900
x+1+x+2+x+3+......+x+9=900
x*9+(1+2+3+...+9)=900
x*9+45=900
x*9=900-45
x*9=855
x=855:9
x=95
3:
Tuổi con là:
24:(3-1)x1=12(tuổi)
Tuổi cha là:
12x3=36(tuổi)
Tuổi ông là:
36x2=72(tuổi)
ĐS:..........
4:
a)S ABC=15x20:2=150(cm2)
b)BD=15:3=5(cm)
AD=15-5=10(cm)
Ta có:
5xDE:2=S BDE
DEx10:2=S ADE
20xAD:2=S AEC
mà S BDE+S ADE +S AEC=S ABC
nên:
5xDE:2+DEx10:2+20x10:2=150
(5xDE+DEx10+20x10):2=150
5xDE+DEx10+20x10=150x2
5xDE+DEx10+200=300
5xDE+DEx10=300-200
5xDE+DEx10=100
DEx(5+10)=100
DEx15=100
DE=100:15
DE=6.666
S ADE=10x6,666:2=33,33(cm2)
ĐS:.......
.......
5:
Tổng 3 số là:
4000x3=12000
0,5=1/2=2/4
Ta có sơ đồ:
STN:|-------------|--------------|-------------|
ST2: |-------------|--------------|-------------|-----------| =12000
ST3: |-------------|--------------|-------------|-----------|------------|
STN=12000:(3+4+5)x3=3000
ST2=12000:(3+4+5)x4=4000
ST3=12000:(3+4+5)x5=5000
ĐS:...........
