tinh nhanh
1+2+3-4+5+6-7+8+9=?
Tính nhanh
1×2×3×4×5×6×7×8×9×10×11×12×13×14×15×16×17×18×19×20
tinh nhanh:1+9+2+8+3+7+4+6+5+5+6+4+7+3+8+2+9+1
1 + 9 + 2 + 8 + 3 + 7 + 4 + 6 + 5 + 5 + 6 + 4 + 7 + 3 + 8 + 2 + 9 + 1
= 10 + 10 + 10 +10 +10 + 10 + 10 + 10 + 10
= 10 x 9
= 90
1+9+2+8+3+7+4+6+5+5+6+4+7+3+8+2+9+1
=10+10+10+10+10
=50
tinh nhanh A = 1*2+2*3+3*4+4*5+5*6+6*7+7*8+8*9+9*10
Đặt S = 1 x 2 + 2 x 3 + ..... + 9 x 10
3S = 1 x 2 x (3-0) + 2 x 3 x (4 - 1) + ....... + 9 x 10 x (11 - 8)
3S = 1 x 2 x 3 + 2 x 3 x 4 - 1 x 2 x 3 + ......... + 9 x 10 x 11 - 8 x 9 x 10
3S = ( 1 x 2 x 3 -1 x 2 x 3) + ( 2 x 3 x4 - 2 x 3 x 4) + ..... + (8 x 9 x 10 - 8 x 9 x 10) + 9 x 10 x 11
3S = 9 x 10 x 11 = 990
S = 990 : 3 = 330
Tinh Nhanh:
6 2/7 + 7 3/5 + 8 6/9 + 9 1/4 +2/5 + 5/7 + 1/3 + 3/4 + 1967
tinh nhanh
1/10 nhan 2/9 nhan 3/8 nhan 4/7 nhan 5/6 nhan 6/5 nhan 7/4 nhan 8/3 nhan 9/ 2
1/10x(2/9x9/2)x(3/8x8/3)x(5/6x6/5)=1/10x1x1x1=1/10
Tính nhanh1*3+3*5+5*7+7*9+......+99*101
Đặt A=1x3+3x5+5x7+7x9+...+99x101
6A=6x(1x3+3x5+5x7+7x9+...+99x101)
6A=1x3x6+3x5x6+5x7x6+7x9x6+...+99x101x6
6A=1x3x(5+1)+3x5x(7-1)+5x7x(9-3)+7x9x(11-5)+...+99x101x(103-97)
6A=1x3x5+1x3+3x5x7-3x5+5x7x9-3x5x7+7x9x11-5x7x9+...+99x101x103-99x101x97
6A=3+99x101x103
=>A=\(\frac{\text{3+99x101x103}}{6}\)
bài đó có ở trong sách toán ko hay tự đố ở ngoài vậy?
ds:6
Tinh nhanh
A=1/1*2+1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+1/7*8+1/8*9+1/9*10
A=1/1*2+1/2*3+...+1/9*10
=1-1/2+1/2-1/3+...+1/9-1/10
=(1-1/10)+(1/2-1/2)+...+(1/9-1/9)
=(10/10-1/10)+0+...+0=9/10
\(A=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{9.10}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{9}-\frac{1}{10}=1-\frac{1}{10}=\frac{9}{10}\)
2/5-2/9+2/11/7/5-7/9+7/11:1/3-1/4+1/5/7/6-7/8+7/10:tinh nhanh
thuc hien phep tinh
( 5*4^15*9-4*3^20*8^9) /( 5*2^9*6^19-7*2^29*27^6)
Tách hai dấu ngoặc thành hai vế A và B
Ta có :
A=5.415.99-4.320.89=5.230.318-22.227.320
=229.318(2.5-32)=229.318
B=5.29.619-7.229.276=5.29.219.319-7.229.318
=228.318(5.3-7.2)=228.318
=>A:B=229.318:228:318=2