pistmrstorix

1)√0.81+√36=10(√81/100)+6=10*9/10+6=15

2)√13²-12²=√1*25=√25=5

3)√28-√63+4√7=2√7-3√7+4√7=3√7

4)a+b-(√a-√b)²=a+b(a-2√ab+b)=a+b-a+2√ab-b=2√ab

дальше сам))