Đáp án+giải thích các bước giải:
`b,\frac{45-x}{1963}+\frac{40-x}{1968}+\frac{35-x}{1973}+\frac{30-x}{1978}+4=0`
`(\frac{45-x}{1963}+1)+(\frac{40-x}{1968}+1)+(\frac{35-x}{1973}+1)+(\frac{30-x}{1978}+1)=0`
`\frac{45-x+1963}{1963}+\frac{40-x+1968}{1968}+\frac{35-x+1973}{1973}+\frac{30-x+1978}{1978}=0`
`\frac{2008-x}{1963}+\frac{2008-x}{1968}+\frac{2008-x}{1973}+\frac{2008}{1978}=0`
`(2008-x)(\frac{1}{1963}+\frac{1}{1968}+\frac{1}{1973}+\frac{1}{1978})=0`
`⇒2008-x=0`
(vì `\frac{1}{1963}+\frac{1}{1968}+\frac{1}{1973}+\frac{1}{1978}>0`)
`⇒-x=-2008`
`⇒x=2008`
