다음 적분방정식을 만족시키는 $f \left (t \right)$ 는?

$\displaystyle f \left (t \right) =1+ \cos t+ \int _ {0} ^ {t} {f \left (\tau \right) \sin \left (t- \tau \right) d \tau}$

• ①$f \left (t \right) =t+ \sin t$
• ②$f \left (t \right) =t- \cos t$
• ③$f \left (t \right) =2+t ^ {2}$
• ④$\displaystyle f \left (t \right) =2+ \frac{1} {2} t ^ {2}$