Помогите сделать 1-3

1) r = (a + b - c)/2
a + b - c = 2r
c = a + b - 2r = 20 + 99 - 2*9 = 101

2) m = sqrt(2*4^2 + 2*7^2 - 9^2)/2 = sqrt(32 + 98 - 81)/2 = sqrt(49)/2 = 7/2

3) R = 10,5 : (2*21/34) = 10,5 : 21/17 = 105/10 : 21/17 = 105/10 * 17/21 = 5*17/10 = 85/10 = 8,5