又系一條m2數=.=

john24d2003

(a+2x)^5=a^5+5a^4(2x)+10a^3(2x)^2+...
40a^3=40  ???

vin712

(a+2x)^5=a^5+5a^4(2x)+10a^3(2x)^2+...
40a^3=40  ???
john24d2003 發表於 16-5-2011 01:26

    你可唔可以用英文解一次比我聽-_- 我睇唔明個d系數...正數解...

john24d2003

你可唔可以用英文解一次比我聽-_- 我睇唔明個d系數...正數解...
vin712 發表於 16-5-2011 01:33

  爆開你應該識
say for example,(2+2x^2)^2=4+8x^2+4x^4
x^4 term=4x^4
coefficient of x^4(系數)=4
constant term=4
coefficient of x^3=0 (without x^3 term)

maxycy

Method 1:
(a+2x)^5
=(5C0)(a^5)(2x)^0  +  (5C1)(a^4)(2x)^1  +  (5C2)(a^3)(2x)^2  +  (5C3)(a^2)(2x)^3  +  (5C4)(a^1)(2x)^4  +  (5C5)(a^0)(2x)^5
=a^5  +  (10a^4)x  +  (40a^3)x^2  +(80a^2)x^3  +  80ax^4  +  32x^5

so,
40a^3=40
    a^3=1
         a=1
=====================================================
Method 2:

the (r+1)th term
=(5Cr)(a^r)[(2x)^(5-r)]
=(5Cr)(a^r)[2^(5-r)][x^(5-r)]

To find the coefficient of x^2
5-r=2 , i.e.  r=3

So the coefficient of x^2
=(5C3)(a^3)[2^(5-3)]
=(10)(a^3)(4)
=40a^3

so,
40a^3=40
    a^3=1
         a=1

vin712

爆開你應該識
say for example,(2+2x^2)^2=4+8x^2+4x^4
x^4 term=4x^4
cofficient of x^4(系數)=4
con ...
john24d2003 發表於 16-5-2011 02:01

    喔..thx...其實我今日都問左呀sir..佢解左比我聽係咩黎v_v


        

上完m2堂自不然老師就會比d題目我地返去做a___a,但系有d題目未教過=.=
1.已知在(a+2x)^5的展開式中,x^2的係 ...
_銘記於心` 發表於 16-5-2011 00:21

         假若你仲記得d次方既pattern既話    你應該會搵到40a^3 x^2
         咁姐係40a^3 = 40
                            a=1

   唔明再問or睇5樓@@

john24d2003

Method 1:
(a+2x)^5
=(5C0)(a^5)(2x)^0  +  (5C1)(a^4)(2x)^1  +  (5C2)(a^3)(2x)^2  +  (5C3)(a^2)(2x)^3  ...
maxycy 發表於 16-5-2011 09:22

條題目問到X^2
唔使爆到咁大
有X^2term就可以+...

john24d2003

明白了!但系5c=階x記法?!
_銘記於心` 發表於 16-5-2011 22:36

del...