g′′ continuous,g(0)=0,g′(0)=0,g′′(0)=0
f(x)=g(x)sinx
I: limx→0[g(x)cotx−g(0)cosecx]=f′′(0)
II: f′(0)=g(0)
1. check II: f′(0)
2. evaluate the limit in I
3. compare with f′′(0)
f′(x)=g′(x)sinx+g(x)cosx
f′(0)=g′(0)⋅0+g(0)⋅1=g(0)⇒II TRUE
g(x)cotx−g(0)cosecx=sinxg(x)cosx−g(0)
limx→0sinxg(x)cosx−g(0)=cos0g′(0)cos0−g(0)sin0=10=0
f′′(x)=g′′(x)sinx+2g′(x)cosx−g(x)sinx
f′′(0)=0
lim=0=f′′(0)⇒I TRUE
II (f′(0)=g(0)) does not explain I
Both true; II not a correct explanation⇒(2)