4. If the seventh term of an arithmetic sequence is 55 and the twenty-second term is...
4. If the seventh term of an arithmetic sequence is 55 and the twenty-second term is 145, what is the eighteenth term? 5. The center cable of a suspension bridge forms a parabolic arc. The cable is suspended from the tops of two support towers, 800 feet apart. The tops of the towers are 170 feet above the road and the lowest point of the cable is midway between the towers and ten feet above the road. Find the height of the cable above the road at a dis- tance of 100 feet from a tower. 6. A brick staircase has a total of 30 steps. The bottom step requires 100 bricks. Each successive step requires two less bricks than the prior step. How many bricks are required to build the staircase?