Consider the two forces acting on a person who stands still, namely ,the downward pull of gravity and the upward support of the floor. are these equal or opposite? do they comprise an action-reaction pair? why or why not?

The downward force of your weight on the floor and the upward force of the floor against the bottom of your shoes are an action-reaction pair.

Those two forces against the soles of your shoes are equal and opposite forces.  If they were not equal and opposite, then they would add up to something that would not be zero.  There would be some NET force there, and the soles of your shoes would be accelerating up or down.  Since they're NOT doing that, and you're just standing there motionless, there can't be any net force there.  Those two forces must exactly add up to zero.

two forces such as weight and normal that are in the same body are not the force of action and reaction

Explanation:

Newton's third law says: "If two bodies interact the force exerted by the body 1 on the body 2 is of equal magnitude and opposite to the force exerted by the object 2 on the 1"

Let's analyze this sentence in our case, the bodies are the Earth and the chair

The Earth pulls the body creating its weight at the same time the body pulls the Earth, with an equal force, notice that each force is applied to a different body, so they cannot be added. These are forces of action and reaction

The chair does not accelerate due to its weight because it is supported by the floor, but it creates a very small deformation that creates a response called Normal force to this support, applied to the chair. The force of the deformation is applied to the floor. These are forces of action and reaction.

Therefore two forces such as weight and normal that are in the same body are not the force of action and reaction

pythagoras' theorem

square root (-5^2+9^2)

square root (25+81)

square root (106)

angle is tan^-1(9/5)

neg x, pos y