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# EdExcel P4 2moro watch

1. Would anyone mind going over the basic principles of differentiation to find tangents in polar co-ordinates, and inequalties?
2. hi there.

differentation and tangents and polars

ok basically, you have two that you use; tangents parallel and perpendicular to the initial line.

for tangents parallel, u can work them out using d/dt(rsint) = 0
so, whatever the equation of the curve u times by sint, differentiate and put equal to 0 (for stationary points).
then work out the values of t and r, accordingly.

for tangents perpendicular you use d/dt(rcost) = 0

inequalities
ok there are two ways to do these; graphically and algebraically.

use graphical for modulus graphs
so, draw the graphs, find the intersections and work out the inequalities

for other graphs use algebra
sort the inequality to one fraction or whatever and find the critical values, that is, where y = 0.

then you need to compare the ranges of critical values

so say you have x = -1, 0, 4 as your critical values, the possible ranges of values of x are

x<-1, -1<x<0, 0<x<4 and x>4
use values in these ranges to work out whether the graph is positive or negative in each of these ranges and depending on what youre looking for, choose the appropriate ranges
y(x)
say you were looking for y>0 [it will always be > or < 0, because you would have taken the other side into y(x)]
then every range that is +ve, those are the ranges you are looking for

good luck

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