18. The base of a straight prism with a height of 7 is a rhombus with a diagonal of 6 cm and 8 cm. Calculate the height of this prism 19. The graph of the function y = - + 2 intersects with the x-axis at (a, 0) and the y-axis at (0, b). Find a + b 20. Find the smallest value of the following expression (x2 + 8x + 23)2 - 28 21. If a2 + 5a = -3, find the value of the expression a2 +- 22. If x + y = 5 and |x - a + 3| + ly + b-13| = 0, find a - b. 23. If b3 = 12, b5 = 48 in a geometric series with positive terms , find the sum of the first five terms of the series. 24. The longest side of a triangle whose interior angles are in the ratio of 1: 2: 3 is equal to 10. Find the radius of the circle circumscribed around this triangle. 25. If lgx = =lga+ 21gc2 - lgb, find x (a > 0,b > 0,c # 0)