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The following letter may be of general interest:

Respected Sir,

I have a confusion about the topic optics. I am very confused about the sign convention used in solving lens problems or spherical mirror problems. Please aware me what is the true sign convention to be used.

Thanking you ...

Answer: There are two kinds of sign conventions:

1. Real-Virtual.
2. Co-ordinate.

Both are correct and can be used. However, co-ordinate sign convention is found easy by most and therefore is recommended.

### $f(x)=x^6+2x^4+x^3+2x+3 $$\mathop {\lim }\limits_{x \to 1} \frac{{{x^n}f(1) - f(x)}}{{x - 1}} = 44$$n=?$

Let $f(x)=x^6+2x^4+x^3+2x+3,x \in R$. Then the natural number n for which $\mathop {\lim }\limits_{x \to 1} \frac{{{x^n}f(1) - f(x)}}{{x - 1}} = 44$ is _ _ _ _ . Solution Since the limit has $\left[ {\frac{0}{0}} \right]$ form, L.H. Rule is applicable. Thus, $\mathop {\lim }\limits_{x \to 1} n{x^{n - 1}}f(1) - f'(x) = 44$ $\therefore nf(1) - f'(1) = 44$ $\therefore n.9 - ({6.1^5} + {8.1^3} + {3.1^2} + 2.1) = 44$ $\Rightarrow 9n - 19 = 44$ $\Rightarrow n=7$