a positive number when decreased by 4 is equal to 21 times the reciprocal of the number . the number is
"sol . let the numbers be x . then , x - 4 = 21 / x ⇔ x 2 - 4 x - 21 = 0 ⇔ ( x - 7 ) ( x + 3 ) = 0 ⇔ x = 7 . answer c"
divide(add(sqrt(add(multiply(4, 21), power(4, const_2))), 4), const_2)