u/Happy_Efficiency_189

I think the answer is meant to be 5/4. But I somehow got 85/4. Which part of my working is wrong?

I redid it and i know how to get the correct answer now, by using another method.

But i am curious why this certain specific working didnt work. Did i make a careless mistake? Did i break a weird rule i didnt know exist?

Im new to calculus and weak in this subject, so please be nice :(

Update: i managed to get it!! I found out that actually sin(5x)/5x as 5x approaches 0 would have been 1, not 5. same thing goes for sin(4x)/4x as 4x approaches 0

Now that honestly got me curious because I remember doing some other questions before, where I made sin(Ax)/Ax as Ax->0 = A, not 1, and i still got it right. I cant exactly remember the question but if i ever see it again, I'll post it here.

Thanks everyone!