In the United States, Congress, Judiciary and President have equal powers. President can veto Congress bills, Congress can override vetoes and judiciary can place any executive order or congress bill under judicial review.This helps maintain a check over power corridors helping in placing a halt on abuse of power. Nixon abused power by using state funds against Democrats his opponents. As a result Congress moved a motion to impeach him, and in fear of impeachment Nixon resigned. So, separation of power helped in maintenance of accountability.
Will explain the other question shortly.

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Verily, His command, when He intends a thing, is only that He says "Be!" - and it is! (Al-Quran)