Verified answer

Solve the system

x^2+y^2=25

3x-y=b

and find the value of b for which there one (double) solution.

2x + 2y dy / dx = 0

2y dy / dx = - 2x

dy / dx = - x / y

3 = - x / ( 3x - b )

9x - 3b = - x

10x = 3b

10x / 3 = b