Question

[thiên nhi]

B=1+\(\frac{3}{2^3}+\frac{4}{2^4}+\frac{5}{2^5}+.....+\frac{100}{2^{100}}\)

CHO BIẾT \(1^2\)+\(2^2\)+\(3^2\)+......+\(10^2\)=\(385\)

TÍNH A = \(2^2\)+\(4^2\)+......+\(20^2\)

giúp mình giải 2 bài này với

Answer 1

[fan FA]

A = 2²+4²+6²+...+20² 
= (1.2)² + (2.2)² + (3.2)² + ... + (10.2)² 
= 2^{2 .}1² + 2².2² + 2².3² + ... + 2² .10² 
= 2² . (1² + 2² + 3² + ... + 10²) 
= 4 . 385 

= 1540

Answer 2

[fan FA]

Đặt A1 = 1/2^1 + 1/2^2 + ... + 1/2^100 
A2 = 1/2^2 + 1/2^3 + ... + 1/2^100 
A3 = 1/2^3 + 1/2^4 + ... + 1/2^100 
.................................... 
................................... 
A100 = 1/2^100 
A = 1/2^1 + 2/2^2 + 3/2^3 + 4/2^4 + ... + 100/2^100 = 
= (1/2^1+1/2^2 +...+ 1/2^100) + (1/2^2+1/2^3 +...+ 1/2^100) + (1/2^3+1/2^4 +...+ 1/2^100) + ... + (1/2^100) = A1 + A2 + A3 + ... + A100 
2^101 A1 = 2^100 + 2^99 + 2^98 + ... + 2 (1) 
2^100 A1 = 2^99 + 2^98 + 2^97 + ... + 1 (2) 
(2) trừ (1) ---> 2^100 A1 = 2^100 - 1 ---> A1 = (2^100 - 1) / 2^100 = 1 - 1/2^100 
Tương tự 
2^101 A2 = 2^99 + 2^98 + 2^97 +...+ 2 (3) 
2^100 A2 = 2^98 + 2^97 + 2^96 +...+ 1 (4) 
(4) trừ (3) ---> 2^100 A2 = 2^99 - 1 ---> A2 = (2^99 - 1) / 2^100 = 1/2 - 1/2^100 
Tương tự 
A3 = 1/4 - 1/2^100 = 1/2^2 - 1/2^100 
A4 = 1/2^3 - 1/2^100 
.................................. 
................................. 
A100 = 1/2^99 - 1/2^100 
Vậy A = A1 + A2 + A3 +...+ A100 = (1 + 1/2 + 1/2^2 + ... + 1/2^99) - 100/2^100 
= 2 A1 - 100/2^100 = 2 - 2/2^100 - 100/2^100 = 2 - 51/2^99 
===========================