Theorem
The second fundamental theorem of calculus states that if f is a continuous function on an interval I containing a and
F(x) = ∫ ax f(t) dt
then F '(x) = f(x) for each value of x in the interval I.
Question 1
Approximate F'(π/2) to 3 decimal places if
F(x) = ∫ 3x sin(t 2) dt
Solution to Question 1:
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Since sin(t 2) is continuous for all real numbers, the second fundamental theorem may be used to calculate F'(x) as follows
F '(x) = sin(x 2)
-
which gives
F '(π/2) = sin( (π/2) 2 ) = 0.624 (3 decimal places)
Question 2
Let
F(x) = ∫ 0x 5 / (3 + 2 e t) dt
b) Calculate F'(0)
b) Show that F(1) < F(4)
Solution to Question 2:
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a) 5 / (3 + 2 e t) is continuous, hence the use of the above theorem gives
F'(x) = 5 / (3 + 2 e x)
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which gives
F'(0) = 5 / (3 + 2 e 0) = 1
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b) A closer look at F'(x) reveals that F'(x) is positive for all real values of x and therefore function F is an increasing one. Hence since 4 > 1 then
F(4) > F(1) or F(1) < F(4)
Question 3
Let
F(x) = ∫ -1x2 1 / (1 + t 2) dt
Find F'(x)
Solution to Question 3:
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Let u = x2. F is now given by
F(u) = ∫ -1u 1 / (1 + t 2) dt
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Use the second fundamental theorem to obtain
dF/du = F'(u) = 1 / (1 + u 2)
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We now use the chain rule of differentiation to write
F'(x) = dF/dx = dF/du . du/dx = 2x * 1 / (1 + x 4)
Question 4
Let
F(x) = ∫ u(x)v(x) f(t) dt
where f is continuous everywhere and u and v are continuous functions of x. Express F'(x) in terms of u', v', u, v and f.
Solution to Question 4:
-
Let a be a real number and write the given integral as the sum (difference) of two integrals as follows
F(x) = ∫ u(x)v(x) f(t) dt
= ∫ u(x)a f(t) dt
+
∫ av(x) f(t) dt
= - ∫ au(x) f(t) dt
+
∫ av(x) f(t) dt
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Since F(x) is the sum of two functions F1 = - ò au(x) f(t) dt and F2 = ò av(x) f(t) dt, then F'(x) is given by
F(x) = F'1(x) + F'2(x)
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We now use the second fundamental theorem to write
dF1/du = - f(u)
dF2/dv = f(v)
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We now use the chain rule of differentiation to write
F'1(x) = dF1/du * du/dx = - f(u) * du/dx
F'2(x) = dF2/dv * dv/dx = f(v) * dv/dx
-
Finally F'(x) is given by
F'(x) = F'1(x) + F'2(x)
= v'(x) * f(v(x)) - u'(x) * f(u(x))
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