Tga+ctga=1/tga+tga=(1+tg²a)/tga
4ctga:(1+tg²a+1)/tga=4ctga*tga/(1+tg²a)=4/(1+tg²a)
4/(1+tg²a)*(1+tg²a)=4
6х+6у=5-2х-у
3х-2у=-3у-3
8х+7y=5
3х+у=-3. *(-7)
8х+7y=5
+ (-21х) -7у=21
-13х=26
х=-2
У=3
1) 7z + 6z - 13 = 130
13z = 117
Z = 9
2) 21t - 4t - 17= 17
17t = 34
T = 2