Rule of thumb for smoothing angle between two lines `shapePath.lineTo` in java/kotlin

Question

I want to make smooth the angle between two mirrored lines shapePath.lineTo(x1,y1) and shapePath.lineTo(x2,y2) by adding an arc of given radius R between that two lines. But I don't know how to work with shapePath.addArc(l, t, r, b, startAngle, SweepAngle). Can you give me a rule of thumb for this to make smooth any similar sharp angle like edge of bottomAppbar of this answer

I did it somehow using cubic Bezier curve shapePath.cubicToPoint(x, y, x, y, z, w). But using this method, an strange dense shadow appear behind the arc part. How can one do that using shapePath.addArc?