математик2011 1. 
=8,528
2.
13/14-2/7 = 13/14-4/14 = 9/14
3.
2 1/4*1 1/4 = 9/4*5/4 = 45/16 = 2 13/16
4.
3,14:5=0,628
5.
75*0,3 = 22,5
6.
35*2/5 = 14
7.
=8
8.
=2
9.
75:0,15 = 500
10.
18:2/3 = 18*3/2 = 27
11.
=(3х)²-(2у)² = 9х²-4у²
12.
=16х²-24ах+9а²
13.
30а²-15ау-0,6ау+0,3у² = 30а²_15,6ау+0,3у²
14.
а=√13²-12² = √25 = 5
15,16,17 - не видно
18.
MN = (8+18)/2 = 13(cм)
S = MN*h = 13*9 = 117(см²)
19.????слишком просто
3a-8b-4a+9b-4 = -a+b-4
20.
S = 8*10*sin30 = 8*10*1/2 = 40(см²)