Why power factor using cos theta angle ?

the active power is expressed in terms of cos theta
the reactive power is expressed in terms of sin theta
reactive power is used to transfer the active power.so it need little amount. we use active power so the power factor using cos theta

as we know the power mainly depends on resistive elements hence from impedance p.f depends on cos angle

among total power some power is useful called active power or real power which interms of cosine angle

p_total=vi
p_active=vi cos(angle)

as power factor is fraction total power is useful i.e ratio of active power to total power so p.f is in cos angle

Literally watt power factor means the factor of real use full power that is going to transmit from the total apparent power produced so that is why it is the ratio of real power to complex power which can b taken as cosine of angle ratio from power triangle. It is the reason why power factor is cos theta.

bcz in impedence triangle(cos=b/h) R(resistance) is in the base and imepedence is hypotaneous.
power factor(cos )=R/Z. THATS WY WE are using cos in the power factor notation.