explain why radius squared divided by y is the y-intercept in the equation of the line tangent to the y-intercept in the equation of the line tangent to the circle x^2+y^2=r^2

like in this example x^2+y^2=17 and the circle meets the tangent at (1,4)What is the equation of the tangent line? he answer is y=(-1/4)x+(17/4)

Why does the (17/4) equal the radius squared by y1?

like in this example x^2+y^2=17 and the circle meets the tangent at (1,4)What is the equation of the tangent line? he answer is y=(-1/4)x+(17/4)

Why does the (17/4) equal the radius squared by y1?