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7 years, 9 months ago
How many isosceles triangles can be formed with no side exceeding 10?excluding equilateral triangles
Answer is 65 ,can anyone explain?
6 years ago
Let x, x, y are integer sides of an isosceles triangle. So for x = 10, y can take values from 1 to 9 satisfying the constraints (So 9 triangles).
Now keep decreasing the value of x by 1 and find how many values of y exist.
For x = 9, y can take values from 1 to 10 except 11 (So 9 triangles).
Similarly, for x = 8, the number of values of y can be 9.
This pattern remains the same until x = 6 (9 values of y)
For x = 5, y cannot be 5 and 10 (so 8 values).
For x = 4, y cannot be 4, 10, 9, 8 (so 6 values).
For x = 3, y cannot be 3, 10, 9, 8, 7, 6 (so 4 values).
For x = 2, y can be 1, 3 (so 2 values).
For x = 1, the only value of y is 1 with the given constraints but this becomes an equilateral triangle so not possible.
So if u sum up the values, it'll be
2+4+…+8+(9∗5) = 20+45 =65
So the total number of triangles possible is 65.