The sinx function has a Maclaurin series expansion
sinx = x - (x^3)/3! +(x^5)/5! - (x^7)/7!+...
This is an alternating series for all nonzero values of x, and the successive terms
decrease in size since |x|<1.
Thus we can use the Alternating Series Estimation Theorem .
The error in approximating sinx by the first three terms of its Maclaurin series is at most
(|x|^7)/(7!)= |x^7|/5040
If -.3≤ x ≤.3, then |x|≤.3 so the error is smaller than
(0.3)^7/ 5040≅ 4.3 (10)^(-8)