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12. (a) Suppose a, b > 0 are constants, and U = arctan - tan c ab a for r E (- ?' 2) Express dy dx as a function of sin r and cosr. (b) Suppose a, b > 0 are constants, and a + btan x y = In a - btan r for re (? ? ) \\+ arctan ( )}. Express dy as a function of sinr and dr COST