угол между векторами a(5; -12) и b(3;y) равен 45.Найти y

1. arccos((5*3+(-12)*y)/(√(5^2+(-12)^2)*√(3^2+y^2))) = 45°;

2. (15+(-12)*y)/(√(25+144)*√(9+y^2)) = cos(45°);

3. (15+(-12)*y)/(√169*√(9+y^2)) = 1/√2;

4. (15+(-12)*y)/(13*√(9+y^2)) = 1/√2;

5. (15+(-12)*y)*√2/(13*√(9+y^2)*√2) = 1*13*√(9+y^2)/(√2*13*√(9+y^2))

6. (15+(-12)*y)*√2 = 1*13*√(9+y^2);

7. (15+(-12)*y)*√2 = 13*√(9+y^2);

8. ((15+(-12)*y)*√2)^2 = (13*√(9+y^2))^2;

9. (15+(-12)*y)^2*(√2)^2 = 13^2*(√(9+y^2))^2;

10. (15+(-12)*y)^2*2 = 169*(9+y^2);

11. (15^2+2*15*(-12)*y+(-12*y)^2)*2 = 9*169+y^2*169;

12. (225+(-360)*y+(-12)^2*y^2)*2 = 1521+y^2*169;

13. (225-360*y+144*y^2)*2 = 1521+y^2*169;

14. 225*2-360*y*2+144*y^2*2 = 1521+y^2*169;

15. 450-720*y+288*y^2 = 1521+y^2*169;

16. 450-1521+288*y^2-169*y^2-720*y = 0;

17. -1071+119*y^2-720*y = 0;

18. 119*y^2-720*y-1071 = 0;

19. y = (-(-720)/2+/-√((-720/2)^2-119*(-1071)))/119;

20. y = (720/2+/-√((-360)^2+127449))/119;

21. y = (360+/-√(129600+127449))/119;

22. y = (360+/-√257049)/119;

23. y = (360+/-507)/119;

24. y1 = (360+507)/119;

25. y1 = 867/119;

26. y1 = 867/17/(119/17);

27. y1 = 51/7;

28. y2 = (360-507)/119;

29. y2 = -147/119;

30. y2 = -147/7/(119/7);

31. y2 = -21/17;

32. arccos((5*3+(-12)*y1)/(√(5^2+(-12)^2)*√(3^2+y1^2)));

33. arccos((5*3+(-12)*(51/7))/(√(5^2+(-12)^2)*√(3^2+(51/7)^2)));

34. 135° ≠ 45°;

35. arccos((5*3+(-12)*y2)/(√(5^2+(-12)^2)*√(3^2+y2^2)));

36. arccos((5*3+(-12)*(-21/17))/(√(5^2+(-12)^2)*√(3^2+(-21/17)^2)));

37. 45° = 45°;

Ответ: -21/17;

Вроде верно...