1/(1*2)+2/(1*2*3)+3/(1*2*3*4)+.+8/(1*2*3*4*5*6*7*8*9)=?

1/(1*2)+2/(1*2*3)+3/(1*2*3*4)+.+8/(1*2*3*4*5*6*7*8*9)=?

(1) Preparatory knowledge: n / [(n + 1)!] = [(n + 1) - 1] / [(n + 1)!] = [1 / N!] - [1 / (n + 1)!] (2) original formula = 1 - (1 / 9!) = (9! - 1) / 9

1+3+5+7+9.+99——（2+4+6+...

Original formula = (1-2) + (3-4) + (5-6) +... + (97-98) + 99
=49×(-1)+99
=99-49
=50

Given that the perimeter of a triangle is 2x & # 178; cm, and that both sides are x + 1cm, X & # 178; - 2x + 1cm, then the third side of the triangle is? Cm

2X²-X-1-X²+2X-1=X²+X-2=(X+2)×（X-1)

When the elevator is stationary, the spring extends 10 cm and when it is moving, it extends 5 cm. Why is it weightless

The definition of weightlessness refers to the fact that an object is only subjected to gravity or the resultant force of gravity greater than external force,
When the elevator is still, the tension of the spring is equal to and balanced with the gravity of the object. When the object is moving, the tension of the spring decreases and is less than the gravity, so it is in the weightless state of pure hand play

The order of mixed decimal operations is the same as that of what

The order of four mixed operations of decimal and (integer) is the same

The energy of a thin disk with mass m, radius R and angular velocity W?

When rotating around the axis perpendicular to the disk center, the moment of inertia is
J=(1/2)mr^2
Rotational kinetic energy EK = (1 / 2) JW ^ 2 = (1 / 4) m (RW) ^ 2

The sequence {an} is an equal ratio sequence with A1 = 4 as the first term, and S3, S2, S4 are equal difference sequences
(1) Finding the general term formula of {an}
(2) Let BN = log2 | an | and tn be the sum of the first n terms of the sequence {1 / BN * BN + 1}

1.2S2=S3+S42S2=S2+A3+S2+A3+A42A3+A4=0A4/A3=-2An=A1×q^n=4×(-2)^(n-1)=(-2)^(n+1)2.|An|=|(-2)^(n+1)|=2^(n+1)Bn=log2|An|=log2(2^(n+1)=n+11/(Bn×B(n+1)=1/((n+1)×(n+1+1))=((n+2)-(n+1))/((n+1)(n+2))=1/(n+...

When the car runs at a constant speed of 36km / h, if the car brakes at a quadratic acceleration of 0.6m/s, what is the speed reduction after 10s and 20s

The initial speed VO = 36km / h = 10m / s, a = 0.6m/s2, according to vt = VO + at, the speed after 10s is V1 = 10 + (- 0.6) × 10 = 4m / s. The time required for the vehicle speed to drop to 0 is 0 = 10 + (- 0.6) t, t = 16.6s, so the speed after 20s is 0

Xiaohong has to walk 10 minutes to get home from school, Xiaohua has to walk 14 minutes, Xiaohua has to walk 1 / 6 more than Xiaohong, Xiaohong has to walk 12 meters more than Xiaohua every minute, Xiaohua has to walk more

Xiaohua travels x meters per minute, so Xiaohong travels x + 12 meters per minute
14x/10*(x+12)=(1+6)/6
84x=70x+840
x=60
60*14=840
If you don't understand, you can ask

1. What are the most important changes during cell division? What are the significance and results of cell division? 2. The growth process of plant cells? 3. What is cell differentiation? What is tissue?
4. What are the main cells of plants? What are the basic tissues of animals? 5. Is the genetic material in the nucleus of the new cells after division half of that of the original cells? 6. What are the reasons for the continuous growth of organisms? & nbsp; & nbsp; & nbsp;
&Don't tell me to look for it in books. The answer is simple;

Cell division increases the number of cells, which is of great significance to the growth and reproduction of organisms
3. The result of gene selective expression is cell differentiation; the collection of cells with similar structure and same function is called tissue
4. Epidermal cells, guard cells, meristematic zone cells, sieve tube cells, duct cells, mesophyll cells, germ cells, somatic cells
There are four basic tissues in animals: epidermis, connective tissue, muscle tissue and nerve tissue