tg5a/1+tg5a +tg5a/1-tg5a=tg5a(1-tg5a+1+tg5a)/1-tg^5a=2tg5a/1-tg^5a=tg10a
2)cos3b-ctg6b•sin3b=cos3b-cos6b•sin3b/sin6b=cos3b-cos6b•sin3b/2sin3b•cos3b=cos3b-cos6b/2cos3b=2cos^3b-(cos^3b-sin^3b)/2cos3b=(cos^3b+sin^3b)2cos3b=1/2cos3b
