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earboth · #3 October 5th, 2007, 12:08 PM

Quote:

Originally Posted by Joyce

...

limx->90'- secX/tanx

what it means by putting positive and negative behind?how to solve by using other way?

Hi,

$\lim_{x \to \frac {\pi}{2}}\left(-\frac{\sec(x)}{\tan(x)}\right)=\lim_{x \to \frac {\pi}2}\left(-\frac{\frac{1}{\cos(x)}}{\frac{\sin(x)}{\cos(x)}}\right)=$ $\lim_{x \to \frac {\pi}2}\left(-\frac{1}{\cos(x)} \cdot \frac{\cos(x)}{\sin(x)}\right)=\lim_{x \to \frac {\pi}2}\left(-\frac{1}{\sin(x)}\right) =$ $-1$

Quote:

Originally Posted by Joyce

what it means by putting positive and negative behind?how to solve by using other way?

I only can guess because I don't know this way of writing: Probably the - or + sign indicates the direction of approach. So 90°- could mean that you approaches the 90° from the left where the values are smaller than 90°. Ask your teacher or try to find an explanation in your textbook.