log 2 根下7/48+log 2 12-1/2log 2 42
=log 2 根下7/48+log 2 12-log 2 根号42
=log 2 [根下7/48 * 12 / 根号42 ]
=log 2 [根下7/(48*42) * 12 ]
=log 2 [1 / 根下2 ]
=log 2 [2^(-1/2)]
=-1/2