# Thread: Finding equation of Tangent.

1. ## Finding equation of Tangent.

The curve y = √8x + 1 and the tangent at the point P(3,5) on the curve. This tangent meets the y-axis. Find the equation of the tangent at P.

Can anyone pls tell me how to solve this? I've tried many times and I got wrong answer :S The answer is supposed to be 4x-5y+13=0 but i got y=4x+7

2. Originally Posted by Berkshire91
The curve y = √8x + 1 and the tangent at the point P(3,5) on the curve. This tangent meets the y-axis. Find the equation of the tangent at P.

Can anyone pls tell me how to solve this? I've tried many times and I got wrong answer :S The answer is supposed to be 4x-5y+13=0 but i got y=4x+7
Show us the work you have done so far and we can help you from there.

3. This is what I did. dy/dx = 4/√8x+1 then I solved it to find the gradient when x=0 so the gradient is 4. so the equation is y=mx + c. so it would be y=4x+c

to find c. I used this (3,5). So I got C = 7. That means y = 4x + 7. Did I use the right way?

4. I finally got the answer. I didn't read the question carefully. The point is given (3,5). To find the gradient of the tangent. The x = 3 not 0. I didn't know where the 0 came from. hehe. The gradient is 4/5 not 4. Then I can solve using this equation y = mx + c

5 = 4/5(3) + c
so c = 13/5 so. the equation of the tangent is y = 4/5x +13/5. Which means 5y=4x+13.