The function f(x)\displaystyle {f{{\left({x}\right)}}} has a domain of all real numbers, is periodic and has a period of p=21\displaystyle {p}={21}. The graph of one complete period is shown below.
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Find the following values:
f(17)=\displaystyle {f{{\left({17}\right)}}}=  
f(36)=\displaystyle {f{{\left({36}\right)}}}=  
f(423)=\displaystyle {f{{\left({423}\right)}}}=