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Maxima and minima problem watch

1. Isn't it if second derivative is +ve then it's minima and if it's -ve then it's maxima? Here's my working, mark scheme says maxima but I got a positive value for f''(x). Please help

2. (Original post by Taffss)
Isn't it if second derivative is +ve then it's minima and if it's -ve then it's maxima? Here's my working, mark scheme says maxima but I got a positive value for f''(x). Please help

Your initial statement is correct. There is an error in your working when you differentiated -12cos2(x)sin(x)
3. (Original post by joostan)
Your initial statement is correct. There is an error in your working when you differentiated -12cos2(x)sin(x)
I rechecked it again and I think it's the sign problem, Instead I should've kept +ve cosx let me check
4. (Original post by joostan)
Your initial statement is correct. There is an error in your working when you differentiated -12cos2(x)sin(x)
I made cosx +ve and got -ve for f''(x) therefore max. Thanks
5. (Original post by Taffss)
I made cosx +ve and got -ve for f''(x) therefore max. Thanks
Well done
NP

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