−2‌cos‌x+(2x−1)(−sin‌x)+cos‌x(2) =(2x2−x)−2‌cos‌x−2x‌sin‌x+sin‌x+2‌cos‌x =2x2−x−2x‌sin‌x+sin‌x =2x(x−sin‌x)−1(x−sin‌x) f′(x)=(2x−1)(x−sin‌x) for x>0,x−sin‌x>0x<0,x−sin‌x<0 for x∈(−∞,0]∪[‌