`1.`
hình ạ
`2.`
`a,7x-8=715`
`7x=723`
`x=723/7`
`b,123-5.(x+4)=38`
`5.(x+4)=85`
`x+4=17`
`x=13`
`c,49.7^x = 2401`
`7^x=49`
`7^x=7^2`
`x=2`
`3.` Anh không biết em thông cảm
`4.`
`A=4+2^2+2^3+2^4+.....+2^{20}`
Đặt `N=2^2+2^3+2^4+....+2^{20}`
`2N=2^3+2^4+2^5+....+2^{21}`
`2N-N=(2^3+2^4+2^5+....+2^{21})-(2^2+2^3+2^4+....+2^{20})`
`N=2^{21}-2^2`
`N=2^{21}-4`
`=>A=N+4`
`A=2^{21}-4+4`
`A=2^{21}`