Cho x, y, b, d ∈ N*. Chứng minh nếu $\frac{a}{b}$ < $\frac{c}{d}$ thì $\frac{a}{b}$ < $\frac{xa+yc}{xb+yd}$ < $\frac{c}{d}$
Cho x, y, b, d ∈ N*. Chứng minh nếu $\frac{a}{b}$ < $\frac{c}{d}$ thì $\frac{a}{b}$ < $\frac{xa+yc}{xb+yd}$ < $\frac{c}{d}$
By Liliana
Ta có :
`a/b < c/d`
`⇒ ad < bc`
`⇒ ady < bcy`
`⇒ ady + abx < bcy + abx`
`⇒ a(bx+dy) < b(ax+cy)`
$⇒ \dfrac{a}{b} < \dfrac{xa+yc}{xb+yd}$ $(*)$
Lại có :
`a/b < c/d`
`⇒ ad < bc`
`⇒ adx < bcx`
`⇒ adx + cdy < bcx + cdy`
`⇒ d(ax+cy) < c(bx+dy)`
$⇒ \dfrac{xa+yc}{xb+yd} < \dfrac{c}{d}$ $(**)$
Từ $(*),(**)$
`⇒ a/b < [xa+yc]/[xb+yd] < c/d`
`⇒ ĐPCM`
Xin hay nhất !