Given, f(x)=cos‌x+cos‌√2‌x ⇒f′(x)=−sin‌x−√2‌sin‌√2‌x ⇒f"(x)=−cos‌x−2‌cos‌√2‌x For extremum value, put f′(x)=0 ⇒−sin‌x−√2‌sin‌√2‌x=0 ⇒x=0 At x=0,f"(x)<0 (maximum) Hence, f(x) is maximum only once.