What is the summation formula of arithmetic sequence

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• 1. If we know that the square of a plus the square of B plus the square of C is equal to ab plus BC plus AC, then the relation between ABC is【
• 2. Who knows the summation formula of second order arithmetic sequence?
• 3. If a is not equal to 0, then the equation of X + the square of Y + ax ay = the image represented by 0 On X-axis symmetry On Y-axis symmetry On the symmetry of the line X-Y = 0 On the symmetry of the line x + y = 0
• 4. Is there a formula for summation of second order arithmetic sequence? Is the latter minus the former is the arithmetic sequence, how to find the sum of the original sequence?
• 5. To solve the equation about X, the square of x minus 2x plus 1 equals K (the square of x minus 1) (k is not equal to 0)
• 6. Given the complex number Z, find Z + Z square + Z three square + Can we use the summation formula of real number equal ratio sequence to sum?
• 7. If the solutions of the equations 2x-y23 and x-2y equal to a satisfy that x + y is greater than 0, then the value range of a is a, and the value range of a is a
• 8. Summation formula of permutation and combination
• 9. 2X + 3Y = k the sum of solutions X and y of the system of linear equations {3x-4y = K + 11} with respect to X and Y is known to be equal to 3, and the value of K is obtained The method is as simple as possible
• 10. What is the formula for horizontal summation?
• 11. Judge whether the triangle whose three sides are 2n ^ 2 + 2n, 2n + 1, 2n ^ 2 + 2n + 1 is a right triangle Judge whether the triangle whose three sides are 2n ^ 2 + 2n, 2n + 1, 2n ^ 2 + 2n + 1 (n is greater than 0) is a right triangle Remember to prove which is the hypotenuse! To prove why that's a hypotenuse
• 12. Let Sn = - 1 + 3-5 + 7 - +(- 1) n (2n-1), then SN=______ ．
• 13. Judge whether the triangle with three sides of n ^ 2-1, 2n, n ^ 2 + 1 (n > 1) is a right triangle? And explain the reason
• 14. Sn = 1 / 2 + 3 / 2 ^ 2 + 5 / 2 ^ 3 +... + (2n-1) / 2 ^ n sum My foundation is not good, please try to be more detailed,
• 15. Try to judge: are triangles with 2n * n + 2n, 2n + 1, 2n * n + 2n + 1 sides right triangles?
• 16. Calculate 1 + (1 + 2) + (1 + 2 + 3) + +（1+2+3+… +n）．
• 17. Prove: square of M - square of N, 2Mn, square of M + square of N, (M and N are natural numbers, and M is greater than N and greater than 0) is the side length of right triangle
• 18. Calculate 1 + (1 + 2) + (1 + 2 + 3) + +（1+2+3+… +n）．
• 19. Is a triangle whose three sides are square of a = m - square of N, B = 2Mn, C = square of M + square of n (M greater than n greater than o) a right triangle
• 20. There are two points a and B on the flat ground. A is to the east of mountain D, B is to the southeast of mountain D, and 300 meters in the direction of 25 ° south by west of A. the elevation of C on the top of mountain a is 30 degrees, so as to calculate the mountain height