Đáp án:
`↓↓`
Giải thích các bước giải:
`a) 1+3+5+...+(2x-1)=225`
`=> (1+2x-1). (2x-1+1)/2 : 2=225`
`=> 2x. x/2=225`
`=> 2x^2=450`
`=> x^2=225`
`=> x=+-15`
`b) 2^x+2^(x+1)+2^(x+2)+...+2^(x+2015)=2^(2019)-8`
`=> 2^x+2^x. 2+2^x. 2^2+...+2^x. 2^(2015))=2^(2019)-8`
`=> 2^x. (1+2+2^2+....+2^(2015))=2^(2019)-8`
`=> 2^x . (2^(2016)-1)=2^(2019)-8`
`=> 2^x=(2^(2019)-8)/(2^(2016)-1)`
`=> 2^x=(2^3. (2^(2016)-1))/(2^(2016)-1)`
`=> 2^x=2^3`
`=> x=3`
$\boxed{\text{Khánh Huyền}}$
