中等MATH-ALGE-008
If x^2 + y^2 = 25 and xy = 6, what is (x + y)^2?
A. 31
B. 19
C. 37✓ 答案
D. 13
解說
(x + y)^2 = x^2 + 2xy + y^2 = 25 + 2(6) = 37. Adding xy only once gives 31. Subtracting xy gives 19. The cross term is 2xy, not xy.
(x + y)^2 = x^2 + 2xy + y^2 = 25 + 2(6) = 37. Adding xy only once gives 31. Subtracting xy gives 19. The cross term is 2xy, not xy.