Why do we use compliment method?

Which of the following is a difference between the systems development life cycle (sdlc) and extreme programming (xp)? the sdlc model has a separate planning and analysis phase, whereas xp combines both the phases into one phase.in the sdlc model, changes cannot be made to a system once it is delivered to the user, whereas the xp method delivers the system to the users and then make changes suggested by the user.the sdlc model develops an entire system at once, whereas xp uses incremental steps to improve an information system's qualitydevelopers following the sdlc model cannot go on to the next phase until the current phase is finished, whereas in xp, developers can move to any phase from the current phase.