Ta có :
`b²=ac=>a/b=b/c` `(1)`
`c²=bd=>b/c=c/d` `(2)`
Từ `(1),(2)=>a/b=b/c=c/d`
`=>[a³]/[b³]=[b³]/[c³]=[c³]/[d³]`
`=>[a³]/[b³]=[b³]/[c³]=[c³]/[d³]=(a³+b³+c³)/(b³+c³+d³)`
`=>a/b=b/c=c/d=(a³+b³+c³)/(b³+c³+d³)`
Vì `b/c=c/d`
`=>d/c=c/b`
`=>a/b=d/c`
`=>a/d=b/c=(a³+b³+c³)/(b³+c³+d³)` `(Đpcm)`