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# C2 Circle Tangents watch

1. Toughie !

Given that the line with the equation y=x+k is a tangent to the circle x²+y²=25. Fine the two possible values ok K
2. +5 and -5.

The circle has centre (0,0), radius 5. The line has a gradient of zero, if you just picture it in your head you can see why.
3. (Original post by Fenchurch)
Toughie !

Given that the line with the equation y=x+k is a tangent to the circle x²+y²=25. Fine the two possible values ok K
x² + (x + k)² = 25

x² + x² + 2kx + k² - 25 = 0

2x² + 2kx + k² - 25 = 0

Discriminant = 4k² - 8(k² - 25) = 0

-4k² + 200 = 0

k² = 50

k = ±√50

k = ±5√2

Hope this helps,

~~Simba
4. The answer is ±5√2 but i dunno how to get there lol
5. (Original post by Fenchurch)
The answer is ±5√2 but i dunno how to get there lol
Check my post ... ±√50 = ±5√2
6. Soz Simba :P didnt see your post above, was just bout to say, n00b me √50 i obv 5√2 :P

Simba is maths god
7. Hehe, no problem , thanks for the compliment !

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