trung bình tuổi 3 bạn kia

\(\frac{7+13+4}{3}\)=8(tuổi)

tuổi Hải là

8+3=11

\(P=\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+...+\frac{2008}{3^{2008}}+\frac{2009}{3^{2009}}\)

\(\Rightarrow3P=1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{2009}{3^{2008}}\)

\(\Rightarrow2P=1+\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^{2008}}-\frac{2009}{3^{2009}}=A-\frac{2009}{3^{2009}}\)

\(A=1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{2007}}+\frac{1}{3^{2008}}\)

\(\Rightarrow3A=3+1+\frac{1}{3}+...+\frac{1}{3^{2007}}\)

\(\Rightarrow2A=3-\frac{1}{3^{2008}}< 3\Rightarrow A< \frac{3}{2}\)

\(\Rightarrow2P=A-\frac{2009}{2^{2009}}< A< \frac{3}{2}\Rightarrow P< \frac{3}{4}\)

gợi ý sai zậy sao làm???????????????????????????????????????????????????????

5 + 5 = 6

tk cho mk nha

rất tiếc vì đa làm sai 

\(A=3\left(1+3\right)+3^3\left(1+3\right)+...+3^{2020}\left(1+3\right)\)

\(=4\left(3+3^3+...+3^{2020}\right)⋮4\)

M=(3+3^2)+(3^3+3^4)+....+(3^99+3^100)

M=3(1+3)+3^3(1+3)+....+3^99(1+3)

M=3.4+3^3.4+....+3^99.4

M=4(3+3^3+....+3^99)

SUY RA M CHI HẾT CHO 4

NHỚ TÍCH MK NHA

Bài làm 

4 + 5 = 9

4+5 = 9 nha bạn

Study well                   

1+2=3

2+3=8

3+4=15

Hỏi: 4+5=24

Ko bt đúng ko?

M=3+3²+3³+3⁴+...+3¹⁰⁰

3M=3(3+3²+3³+3⁴+...+3¹⁰⁰)

3M=3²+3³+3⁴+3⁵+...+3¹⁰¹

3M-M=2M=3²+3³+3⁴+3⁵+...+3¹⁰¹-(3+3²+3³+3⁴+...+3¹⁰⁰)

2M=3²+3³+3⁴+3⁵+...+3¹⁰¹-3-3²-3³-3⁴-...-3¹⁰⁰

2M=3¹⁰¹-3

M=\(\frac{3^{101}-3}{2}\)

=) ta có : M=3+3^2+3^3+3^4+...+3^100

            =)M = 3+3²+3³+3⁴ + 3⁵ x ( 3+3²+3³+3⁴  ) + ....+ 3⁹⁵ x (  3+3²+3³+3⁴ )

           =) M = 120 + 3⁵ x 120 + .... + 3⁹⁵ x 120

vì mỗi SSH đều chia hết cho 12 =) M cũng chia hết cho 3

 vậy M chia hết cho 12 và 3

\(4.x=4\dfrac{3}{4}\\ 4.x=\dfrac{19}{4}\\ x=\dfrac{19}{4}:4\\ x=\dfrac{19}{16}\)

`4 . x = 4 3/4`

`=> 4 . x= (4xx4+3)/4`

`=> 4 . x=19/4`

`=> x= 19/4 : 4`

`=> x= 19/4 xx 1/4`

`=> x= 19/16`

\(4.x=4\dfrac{3}{4}\\ 4.x=\dfrac{19}{4}\\ x=\dfrac{19}{4}:4\\ x=\dfrac{19}{16}.\)