A Female Ghost Makes Me Solve Math Problems or She'll Kill Me

Xia Feng ran into a ghost, but this ghost wasn't anything like the ones he'd imagined...

The scene he imagined:

Person: "Please, I've never wronged you! Please don't kill me!"

Ghost: "Everyone... must die."

What actually happened to Xia Feng:

Xia Feng: "I've never wronged you. Could you please not kill me?"

Blood-soaked female ghost: "Fine, as long as you answer one question."

Xia Feng: "What question?"

Female ghost: "Given that the line y=x+1 is tangent to the curve y=lnx+a, what is the value of a? _____"

Xia Feng: "..."

This novel and its characters are entirely fictional. Any resemblance to actual people or events is purely coincidental. Please do not imitate.