Let a,b and c are rtional numbers which satisfy a-7b+8c=4 and 8a+4b-c=7.Then a^2-b^2+c^2=?

来源:学生作业帮助网 编辑:六六作业网 时间:2024/12/25 23:11:05
Leta,bandcarertionalnumberswhichsatisfya-7b+8c=4and8a+4b-c=7.Thena^2-b^2+c^2=?Leta,bandcarertionalnu

Let a,b and c are rtional numbers which satisfy a-7b+8c=4 and 8a+4b-c=7.Then a^2-b^2+c^2=?
Let a,b and c are rtional numbers which satisfy a-7b+8c=4 and 8a+4b-c=7.Then a^2-b^2+c^2=?

Let a,b and c are rtional numbers which satisfy a-7b+8c=4 and 8a+4b-c=7.Then a^2-b^2+c^2=?

a-7b+8c=4 (1)
8a+4b-c=7 (2)
(1)+(2)*8 可得: 65a+25b=60
化简得13a+5b=12 -->a=(12-5b)/13
(1)*8-(2)得-60b+65c=-25
化简得12b-13c=5 --->c=(12b+5)/13
将a,c,代入a²-b²+c²=((12-5b)/13)²+((12b+5)/13)²-b²
=((12-5b)²+(12b+5)²)/13²-b²
=(12²+(5b)²+(12b)²+5²)/13²-b²
=(12²+5²+(12²+5²)b²)/13²-b²
=1+b²-b²
=1

由a-7b+8c=4得a=7b-8c+4 ……(1)代入8a+4b-c=7得b=(13c-5)/12 ……(2)将(2)代入(1)式得a=(13-5c)/12故a^2-b^2+c^2=(13c-5)^2/144-(13-5c)^2/144+c^2=(8c+8)*(18-18c)/144+c^2=1-c^2+c^2=1

Let Ben and Tom ( )good friends.A is B are C be D will be ( )Let john and jack______our friends.A.be B.to be C.are D.is Let you and _____good friends.A.I am B.me be C.me are D.I are(选择 Let _____ help you ,Mr.Hu A.Lucy and me B.I and Lucy C.me and Lucy D.Lucy and I( )Let _____ help you ,Mr.HuA.Lucy and me B.I and Lucy C.me and Lucy D.Lucy and I( )____ are middle school students.A.They,we and you B.We ,you and they C.You ,we and they Let is ( )pen friends.A:be B:are C:is there are few ___in the fridge .let's go and bu some peas and carrots and cabbage.a:vegetables B:fruit C:meat D:eggs Let a,b and c are rtional numbers which satisfy a-7b+8c=4 and 8a+4b-c=7.Then a^2-b^2+c^2=? Tom,Let,s play__that ball.A.with B./ C.and Let you and_____good friends.And let_____help you.A.my are;I B.I am;I C.me be;me d.I are;me(选择并说明) let you and ( )friends.A I am B I are Cme are D me be There are few ____ in the fridge.Let's go and buy some pears,carrots and cabbages.A vegetables B fruit C meat D flowers There are few ________ in the fridge.Let’s go and buy some peas, carrots and cabbages.A. vegetables B. fruit C. meat D. eggs there are only _ oranges at home .Lets go and buy some.A.few B.little C.a little D.a few必须详细说为什么 1.( )fathes are at my birthday party.A.Tom and Paul's B.Tom's and Paul C.Tom's and Paul's2.Let ( )have a look at ( ) photoA.she,he B.her,him C.her,his Some chicken and a bottle of beer for me,please.(回答)回答:A.Here you are B.Good idea C.Let me see Let you and___friends.A,I am B,I are C,me are D,me be Let us ____good friends and always help each other.A.to be B.are C.be D.get回答写理由快 Thet are playing under the tree.Let's go and ____ them.A.join in B.take part in C.join D.joining请说明为什么!感激不尽!