+$\frac{-6}{căn7+3}$ +$\frac{8}{căn7-3}$ +$\frac{12}{căn7}$
2 câu trả lời
`(-6)/(sqrt7+3)+8/(sqrt7-3)+12/sqrt7`
`=(-6sqrt7(sqrt7-3))/(sqrt7(sqrt7+3)(sqrt7-3))+(8sqrt7(sqrt7+3))/(sqrt7(sqrt7+3)(sqrt7-3))+(12(sqrt7+3)(sqrt7-3))/(sqrt7(sqrt7+3)(sqrt7-3))`
`=(-42+18sqrt7+56+24sqrt7+12(7-9))/(sqrt7(7-9))`
`=(42sqrt7+14-24)/(sqrt7(7-9))`
`=(42sqrt7-10)/(-2sqrt7)`
`=(2(21sqrt7-5))/(-2sqrt7)`
`=-(21sqrt7-5)/sqrt7`
`=(-sqrt7(21sqrt7-5))/(sqrt7.sqrt7)`
`=(-147+sqrt7)/7`
Đáp án:
`(-6)/(\sqrt{7} + 3) + 8/(\sqrt{7} - 3) + 12/(\sqrt{7}) = (-147 +5 \sqrt{7})/7`
Giải thích các bước giải:
`(-6)/(\sqrt{7} + 3) + 8/(\sqrt{7} - 3) + 12/(\sqrt{7})`
` = (-6 . \sqrt{7} . (\sqrt{7} - 3) + 8 . \sqrt{7} . (\sqrt{7} + 3) + 12 (\sqrt{7} + 3)(\sqrt{7}- 3) )/( \sqrt{7} (\sqrt{7} - 3)(\sqrt{7} + 3))`
` = ( - 6 . \sqrt{7} . \sqrt{7} + 6 . 3 . \sqrt{7} + 8. \sqrt{7} . \sqrt{7} + 8 . 3 . \sqrt{7} + 12 . [(\sqrt{7})^2 - (\sqrt{3})^2])/( \sqrt{7} . [ (\sqrt{7})^2 - 3^2])`
`= ( - 6 . 7 + 18 \sqrt{7} + 8 .7 + 24 \sqrt{7} + 12 (7-9))/(\sqrt{7} (7-9))`
`= (-42 + 18 \sqrt{7} + 56 + 24 \sqrt{7} - 24 )/( -2 \sqrt{7})`
` = (42\sqrt{7} -10 )/(-2 \sqrt{7})`
` = (2 (21 \sqrt{7} -5 ))/(- 2 \sqrt{7})`
` = ( - (21 \sqrt{7} - 5 ))/(\sqrt{7})`
` = (- \sqrt{7} . 21 \sqrt{7} + \sqrt{7} . 5)/( (\sqrt{7} . \sqrt{7})`
` = (-147 +5 \sqrt{7})/7`