cho 2/c = 1/a +1/b ( a,b,c khác 0 ; b khác c) . CMR : a-c/a = c-b/b 27/11/2021 Bởi Anna cho 2/c = 1/a +1/b ( a,b,c khác 0 ; b khác c) . CMR : a-c/a = c-b/b
Ta có : $\frac{2}{c}$ = $\frac{1}{a}$ + $\frac{1}{b}$ ⇒ $\frac{2}{c}$ = $\frac{a+b}{ab}$ ⇔ 2ab = c ( b + a ) ⇒ b ( a – c ) – a ( c – b ) = ba – bc – ac + ab = ( ba + ab ) – bc – ac = 2ab – bc – ac = c ( b + a ) – bc – ac = cb + ca – bc – ac = ( cb – bc ) + ( ca – ac ) = 0 + 0 = 0 vì b ( a – c ) – a ( c – b ) = 0 ⇒ b ( a – c ) = a ( c – b ) ⇒ $\frac{a-c}{a}$ = $\frac{c-b}{b}$ ( ĐPCM ) Bình luận
Ta có : $\frac{2}{c}$ = $\frac{1}{a}$ + $\frac{1}{b}$
⇒ $\frac{2}{c}$ = $\frac{a+b}{ab}$
⇔ 2ab = c ( b + a )
⇒ b ( a – c ) – a ( c – b )
= ba – bc – ac + ab
= ( ba + ab ) – bc – ac
= 2ab – bc – ac
= c ( b + a ) – bc – ac
= cb + ca – bc – ac
= ( cb – bc ) + ( ca – ac )
= 0 + 0
= 0
vì b ( a – c ) – a ( c – b ) = 0
⇒ b ( a – c ) = a ( c – b )
⇒ $\frac{a-c}{a}$ = $\frac{c-b}{b}$ ( ĐPCM )