Oblicz a,b oraz c jeżeli wiesz,że:
1)W(x)=
W(-2)=-16
W(0)=-4
W(1)=-1
2)W(x)=
W(0)=-1
W(2)=4
W(4)=-5
3)W(x)=
W(-1)=9
W(0)=-3
W(3)=6
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1)
W(x) = 2 x^4 + a x^3 + 2 x^2 + b x + c
W(-2) =2*(-2)^4 + a*(-2)^3 + 2 *(-2)^2 - 2 b + c = - 16
W(0) = c = - 4
W(1) = 2 *1^4 + a*1^3 + 2*1^2 + b + c = - 1
czyli
32 - 8a + 8 - 2 b + c = - 16
c = - 4
2 + a + 2 + b + c = - 1
------------------------------
- 8 a - 2 b = - 16 - 40 + 4
c = - 4
a + b = -1 - 4 + 4
------------------------
- 8 a - 2 b = - 52
c = - 4
a + b = - 1 = > b = -1 - a
--------------------------------
- 8 a - 2*( -1 -a) = - 52
c = - 4
-----------
- 8 a + 2 + 2 a = - 52
- 6 a = - 52 - 2 = - 54
a = 9
====
b = - 1 - 9 = - 10
=================
c = - 4
=======
Odp. a = 9, b = - 10, c = - 4
===============================
z.2
W(x) = (-1/8) x^3 + a x^2 + b x - c
W(0) = - c = - 1 => c = 1
W(2) = (-1/8)*2^3 + a* 2^2 + b*2 - 1 = - 1 + 4a + 2 b - 1 = 4a + 2 b - 2 = 4
W(4) = ( -1/8)*4^3 + a*4^2 + b*4 - 1 = - 16 + 16 a + 4 b - 1 = 16a + 4 b - 17 = - 5
zatem
c = 1
4 a + 2 b = 6 / *(-4)
16 a + 4 b = 12
----------------------
- 16 a - 8 b = - 24
16 a + 4 b = 12
-------------------- dodajemy stronami
- 4 b = - 12 / : (-4)
b = 3
=====
4a = 6 - 2 b = 6 - 2*3 = 0
a = 0
====
Odp. a = 0, b = 3, c = 1
============================
3)
W(x) = x^4 + (1/3) x^3 - a x^2 - b x + c
w(-1) = (-1)^4 + (1/3)*(-1)^3 - a*(-1)^2 - b*(-1) + c = 9
W( 0) = c = - 3
W(3) = 3^4 + (1/3)*3^3 - a*3^2 - b*3 + c = 6
--------------------------------------------------
1 - 1/3 - a + b - 3 = 9
81 + 9 - 9a - 3 b - 3 = 6
-------------------------------
- a + b = 11 1/3 / * 3
- 9 a - 3 b = - 81
--------------------
- 3 a + 3 b = 34
- 9a - 3 b = - 81
-------------------- dodajemy stronami
- 12 a = - 47
a = 3 11/12
==============
b = 11 1/3 + a = 11 1/3 + 3 11/12 = 11 4/12 + 3 11/12 = 14 15/12 = 15 1/4
b = 15 1/4
=============
c = - 3
========
Odp. a = 3 11/12, b = 15 1/4, c = - 3
========================================