Let N be the set of natural numbers. Suppose f:N→N is a function satisfying the following conditions (a)f(m+n)=f(m)+f(n)(b)f(2)=2 The value of 1720∑k=1f(k) is
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Solution
f:N→Nf(m+n)=f(m)+f(n) Putting m=0,n=0, we get f(0)=2f(0)⇒f(0)=0 m=n=1, we get f(2)=2f(1)⇒f(1)=1 m=2,n=1, we get f(3)=f(1)+f(2)=3 Similarly, f(4)=4 Therefore, f(x)=x Now, 1720∑k=1f(k)=1720∑k=1k=20×217×2=30