Thursday, January 5, 2012

What are the coefficients of the function f(x) = mx^4 + nx^2 +p, if f(0)=4,f'(1)=14 and definite integral of f(x), x=0 to x=1 is 6?

To calculate f(0), we'll substitute x by 0 in the
expression of f(x):


f(0) = m*0^4 + n*0^2
+p


f(0) = p


But f(0) = 4 (from
enunciation) => p = 4


Now, we'll calculate
f'(x):


f'(x) = (mx^4 + nx^2
+p)'


f'(x) = 4mx^3 + 2nx


But
f'(1) = 14


We'll calculate f'(1) substituting x by 1 in the
expression of the first derivative:


f'(1) = 4m*1^3 +
2n*1


f'(1) = 4m + 2n


But f'(1)
= 14 => 4m + 2n = 14


We'll divide by
2:


2m +  n = 7


n = 7 – 2m
(1)


Now, we'll calculate the definite
integral:


Int f(x) dx = Int (mx^4 + nx^2 +p)dx = F(1) -
F(0)


Int (mx^4 + nx^2 +p)dx = mInt x^4dx + nInt x^2dx +
pInt dx


Int (mx^4 + nx^2 +p)dx =  m*x^5/5 + n*x^3/3 +
px


F(1) = m/5 + n/3 + p


F(0) =
0


But Int f(x)dx = 6 =>  m/5 + n/3 + p =
6


3m + 5n + 15p = 90


But p =
4=> 3m + 5n = 90 - 60


3m + 5n = 30
(2)


We'll substitute (1) in
(2):


3m + 5(7 – 2m) = 30


3m+
35 – 10m = 30


-7m = -5


m =
5/7


n = 7 – 10/7


n =
39/7


The function f(x) is:
f(x) = (5x^4)/7 + (39x^2)/7 + 4

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