1) (1 - sin²a)/cos²a=cos²a/cos²a=1
2) ( cos²a - 1)/ sin²a=-(1-cos²a)/ sin²a=-sin²a/sin²a=-1
15x=32x
15x-32x=0
-17x=0
x=0
-
V = 20 - 2,5t
2,5t = 20-v
t = (20-v)/2,5
t = (20-v) * 2/5
t = 8-0,4v
3^-11*7^-12:21^-12= ((3*7)^-11*7^-1):21^-12=(21^-11*7^-1)/21^-12=21/7=3