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# Sub in x or y? Tweet

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1. Sub in x or y?
for this q, to find the value of t, how do you know you need to sub 4 into 8cost rather than 2root 3 in the y equation to get t?

Thanks
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2. Re: Sub in x or y?
(Original post by Cleoleo)
for this q, to find the value of t, how do you know you need to sub 4 into 8cost rather than 2root 3 in the y equation to get t?

Thanks
You need to find a value of t that gives the right value for both x and y. So check both.
3. Re: Sub in x or y?
Alternatively, you can look at the graph.

y=4sin(2t) intersects y=2sqrt(3) at two points when 0≤t≤pi/2.
The first (smaller x) is the one for point P, while the second (larger x) is the one for the other point.

x=8cos(t), 0≤t≤pi/2
As t goes from 0 to pi/2, cos(t) goes from 1 to 0. Thus the smaller x has the larger t, so pick the larger value out of the two for t.
Last edited by aznkid66; 20-06-2012 at 16:34.