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Computing Integrals

Addition and Multiplication by a Constant



The natural rules for the definite integral of sums and constant multiplies of functions, i.e.

sumrule, constmult

(f (x) + g(x))dx   = f (x)dx + g(x)dx  
cf (x)dx   = c f (x)dx  

follow (by the Fundamental Theorem of Calculus) from the similar rules for antiderivatives, as we know prove.

Let F(x) and G(x) be two functions with F'(x) = f (x) , G'(x) = g(x) . We know by the addition rule for derivatives that

F(x) + G(x) = [F(x) + G(x)]    

Writing this in terms of f and g yields

f (x) + g(x) = [ f (x)dx + g(x)dx]    

As functions of b , the left and right hand sides of @@the sum rule@@ are antiderivatives of the two expressions above, so they differ by a constant. This constant must be zero, however, since the integrals are equal (both zero) for b = a , and the sum rule is proved.

Similarly, if c is a constant, we know that

c F(x) = [cF(x)]    


cf (x) = [c f (x)dx]    

As before, the @@constant multiple rule@@ asserts the equality of antiderivatives of these two expressions that agree for one value of b . Therefore the antiderivatives are equal, and the rule follows.

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