Không quy đồng mẫu , hãy tính tổng sau :
A = $\frac{5}{2.1}$ + $\frac{4}{1.11}$ + +$\frac{3}{11.2}$ +$\frac{1}{2.15}$ +$\frac{13}{15.4}$
Không quy đồng mẫu , hãy tính tổng sau :
A = $\frac{5}{2.1}$ + $\frac{4}{1.11}$ + +$\frac{3}{11.2}$ +$\frac{1}{2.15}$ +$\frac{13}{15.4}$
`A=5/(2.1)+4/(1.11)+3/(11.2)+1/(2.15)+13/(15.4)`
`A/7=5/(2.7)+4/(7.11)+3/(11.14)+1/(14.15)+13/(15.28)`
`A/7=1/2-1/7+1/7-1/11+1/11-1/14+1/14-1/15+1/15-1/28`
`A/7=1/2-1/28`
`A/7=13/28`
`A=13/4 `
A = $\frac{5}{2.1}$ + $\frac{4}{1.11}$ + $\frac{3}{11.2}$ + $\frac{1}{2.15}$ + $\frac{13}{15.4}$
$\frac{A}{7}$ = $\frac{5}{2.7}$ + $\frac{4}{7.11}$ + $\frac{3}{11.14}$ + $\frac{1}{14.15}$ + $\frac{13}{15.28}$
$\frac{A}{7}$ = $\frac{1}{2}$ – $\frac{1}{7}$ + $\frac{1}{7}$ – $\frac{1}{11}$ + $\frac{1}{11}$ – $\frac{1}{14}$ + $\frac{1}{14}$ – $\frac{1}{15}$ + $\frac{1}{15}$ – $\frac{1}{28}$
$\frac{A}{7}$ = $\frac{1}{2}$ – $\frac{1}{28}$
$\frac{A}{7}$ = $\frac{13}{28}$
A = $\frac{13}{28}$ . 7
A = $\frac{13}{4}$
Vậy A = $\frac{13}{4}$