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just write it out x+(x+4)+(x+8 )...+(x+36)=80
solve 10x+180=80, x=-10
for 40 times, 40(-10)+720=320.
or knowing that sum of 10 items is 80, multiply by 4 to solve for 40th term, 80*4=320
- by seweryn
Fri Jul 18, 2008 10:59 am- Forum: Problem Solving
- Topic: Sequences
- Replies: 6
- Views: 3093
incorrect assumption by jslavi
You cannot assume you will have two isoceles triangles after the imaginary line draw between the points, if this were the case then for the triangle on the left, you would not have two known sides of radical 3 and 1, using the theory of Pythagorus, we know that the length of the hypotenuse for the l...
- by seweryn
Wed Jul 16, 2008 9:16 am- Forum: Problem Solving
- Topic: Prep- Geometry
- Replies: 6
- Views: 2255