Answer: C. 1998, 2000, 2002
Equation of an ellipse
→having center (0,0) , vertex (
and covertex 
and focus 
is given by:
As definition of an ellipse is that locus of all the points in a plane such that it's distance from two fixed points called focii remains constant.
Consider two points (a,0) and (-a,0) on Horizontal axis of an ellipse:
Distance from (a,0) to (c,0) is = a-c = 
Distance from (-a,0) to (c,0) is = a + c = 
a -c + a +c
= a + a
= 2 a →(Option A )
The price for each instructor will be the same at 3 hours. How I determined this answer:
First off, you need to add the initial price and hourly price for each person together, so you already know how much it will cost for 1 hour, including the initial fee. Here's how you do it:
Ieda: $11.00 (hourly price) + $8.50 (initial fee) = $19.50 (for 1 hour)
Thanh: $10.50 (hourly price) + $10.00 (initial fee) = $20.50 (for 1 hour)
Now that you have the price for 1 hour including the initial fee, now you need to find the price for each hour after that. Here's how I did that:
I created a graph that looked like this:
Hours: 1 2 3
Ieda: 19.50 30.50 41.50
Thanh: 20.50 31.00 41.50
Here's how I figured out the price for each hour:
Ieda:
Hour 1 (including initial price):
$11.00 + $8.50 = $19.50
Hour 2 (excluding initial price): Only add the hourly price after Hour 1!
$19.50 + $11.00 = $30.50
Hour 3 (excluding initial price):
$30.50 + $11.00 = $41.50
Thanh:
Hour 1 (including initial price):
$10.50 + $10.00 = $20.50
Hour 2 (excluding initial price):
$20.50 + $10.50 = $31.00
Hour 3 (excluding initial price):
$31.00 + $10.50 = $41.50
So, looking at the graph, their prices are the same once each instruction reaches 3 hours. ($41.50)
I hope I was able to help you! :)
