Differentiation and integration are opposite process. To find the particular function from the derivation, we have to integrate the function.
Question 1 :
If f'(x) = 4x - 5 and f(2) = 1, find f(x)
Solution :
f'(x) = 4x - 5
∫f'(x) = ∫(4x - 5) dx
= (4x2/2) - 5x + c
f(x) = 2x2 - 5x + c ----(1)
Given that :
f(2) = 1
Instead of x, let us apply 2 in (1)
f(2) = 2(2)2 - 5(2) + c
1 = 2 (4) - 10 + c
1 = 8 - 10 + c
1 = -2 + c
c = 1 + 2
c = 3
By applying the value of c in (1)
f(x) = 2x2 - 5x + 3
Hence the required function is 2x2 - 5x + 3.
Question 2 :
If f'(x) = 9x2 - 6x and f(0) = -3, find f(x)
Solution :
f'(x) = 9x2 - 6x
∫f'(x) = ∫(9x2 - 6x) dx
= (9x3/3) - (6x2/2) + c
f(x) = 3x3 - 3x2 + c ----(1)
Given that :
f(0) = -3
Instead of x, let us apply 0 in (1)
f(0) = 3(0)3 - 3(0)2 + c
-3 = 0 - 1 + c
-3 = c
c = -3
By applying the value of c in (1)
f(x) = 3x3 - 3x2 - 3
Hence the required function is 3x3 - 3x2 - 3.
Question 3 :
If f''(x) = 12x - 6 and f(1) = 30, f'(1) = 5 find f(x)
Solution :
f''(x) = 12x - 6
∫f''(x) = ∫(12x - 6) dx
= (12x2/2) - 6x + c
f'(x) = 6x2 - 6x + c1 ----(1)
∫f'(x) = ∫(6x2 - 6x + c1) dx
∫f'(x) = ∫(6x2 - 6x + 5) dx
f'(1) = 5
Apply x = 1 in (1)
f'(1) = 6(1)2 - 6(1) + c1
5 = 6 - 6 + c1
c1 = 5
f(x) = (6x3/3) - (6x2/2) + 5x + c2
f(x) = 2x3 - 3x2 + 5x + c2 -------(2)
f(1) = 30
Apply x = 1 in (2)
f(1) = 2(1)3 - 3(1)2 + 5(1) + c2
30 = 2 - 3 + 5 + c2
30 = 4 + c2
c2 = 30 - 4
c2 = 26
By applying c2 = 26 in (2), we get
f(x) = 2x3 - 3x2 + 5x + 26
Hence the required function is f(x) = 2x3 - 3x2 + 5x + 26.
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