log 3 21 = log 3 (3*7) = 1 + log 3 7
log 3 21 - 1 = log 3 7
log 3 7 + log 3 5= log 3 35 = 0,5*log (sqrt3) 35; log (sqrt3) 35 = 2*(log 3 7 + log 3 5) = 2*(log 3 21 - 1 + log 3 5)=2*(a+b-1)
ответ: 2*(a+b-1)
