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Citrus2011 [14]
3 years ago
15

Consider two functions f and g on [1, 8] such that integral^8_1 f(x) dx = 9, integral^8_1 g(x) dx = 5, integral^8_5 f(x) dx = 4,

and integral^5_1 g (x) dx = 3. Evaluate the following integrals. a. integral^5_1 2f(x) dx = (Simplify your answer.) b. integral^8_1 (f(x) - g (x)) dx = (Simplify your answer.) c. integral^5_1 (f (x) - g (x)) dx = (Simplify your answer.) d. integral^8_5 (g(x) - f(x)) dx = (Simplify your answer.) e. integral^8_5 7g(x) dx = (Simplify your answer.) f. integral^1_5 3f(x) dx = (Simplify your answer.)
Mathematics
1 answer:
Gelneren [198K]3 years ago
4 0

I'll abbreviate the definite integral with the notation,

I(f(x),a,b)=\int_a^bf(x)\,\mathrm dx

We're given

  • I(f,1,8)=9
  • I(g,1,8)=5
  • I(f,5,8)=4
  • I(g,1,5)=3

Recall that the definite integral is additive on the interval [a,b], meaning for some c\in[a,b] we have

I(f,a,b)=I(f,a,c)+I(f,c,b)

The definite integral is also linear in the sense that

I(kf+\ell g,a,b)=kIf(a,b)+\ell I(g,a,b)

for some constant scalars k,\ell.

Also, if a\ge b, then

I(f,a,b)=-I(f,b,a)

a. I(2f,1,5)=2I(f,1,5)=2(I(f,1,8)-I(f,5,8))=2(9-4)=\boxed{10}

b. I(f-g,1,8)=I(f,1,8)-I(g,1,8)=9-5=\boxed{4}

c. I(f-g,1,5)=I(f,1,5)-I(g,1,5)=\dfrac{I(2f,1,5)}2-I(g,1,5)=10-3=\boxed{7}

d. I(g-f,5,8)=I(g,5,8)-I(f,5,8)=(I(g,1,8)-I(g,1,5))-I(f,5,8)=(5-3)-4=\boxed{-2}

e. I(7g,5,8)=7I(g,5,8)=7(5-3)=\boxed{14}

f. I(3f,5,1)=3I(f,5,1)=-3I(f,1,5)=-\dfrac32I(2f,1,5)=-\dfrac32(10)=\boxed{-15}

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