Why the addition of a risk-free asset will lower the overall portfolio risk and raise return to risk ratio?

Why the addition of a risk-free asset will lower the overall portfolio risk and raise return to risk ratio?

Some companies like Facebook, Google, and Apple do not use any long-term debt or 0% LT debt. Does it make financial sense for such companies to use no debt in their capital structure in line with he signalling theory?

Why do stock analysts reach different conclusion on stock valuation and price targets?