Question

So sánh 

a/40²⁰và3^{30 ; b}/7²¹và 8^{20 ;}c/ (1/3+1/3²+1/3^3+..........+1/3⁹⁹)và 1/2

Answer 1

a/ 40^20=40^2.10=1600^10

3^30=3^3.10=27^10

vì 1600^10>27^10 nên 40^20>3^30

Answer 2

a) 40^20=(4^2)^10=16^10

30^30=(3^3)^10=27610

Vì 16<27=>16^10<27^10 hay 4^20<3^30

b) mk chịu

c) Đặt A= 1/3+1/3^2+1/3^3+...+1/3^99

=>3A=3( 1/3+1/3^2+1/3^3+...+1/3^99)

=>3A=1+1/3+1/3^2+...+1/3^98

=>3A-A=(1+1/3+1/3^2+...+1/3^98)-(1/3+1/3^2+1/3^3+...+1/3^99)

=>2A=1-1/3^99

=>A=(1-1/3^99)/2

=>A=1/2 - (1/3^99)/2 < 1/2=>a<1/2