MathematicsEasy110×since 2002Q3856If f:R→Rf:R \to Rf:R→R satisfies fff(x + y) = fff(x) + fff(y), for all x, y ∈\in∈ R and fff(1) = 7, then ∑r=1nf(r)\sum\limits_{r = 1}^n {f\left( r \right)}r=1∑nf(r) isA7n(n+1)2{{7n\left( {n + 1} \right)} \over 2}27n(n+1)B7n2{{7n} \over 2}27nC7(n+1)2{{7\left( {n + 1} \right)} \over 2}27(n+1)D7n+(n+1)7n + \left( {n + 1} \right)7n+(n+1)Check answerSkip