sin39π/2 - cos7π/2=sin(40π/2 -π/2) - cos(8π/2-π/2)=sin(20π- π/2) - cos(4π - π/2)=

= - sinπ/2 - cosπ/2= - 1-0= -1. (используй формулы приведения либо периодичность)