數學問題 (一題only)

Given sinx + cosx = 3/5
(sinx+cosx)^2 = 9/25
sinx^2(x) + 2sinxcosx + cos^2(x) = 9/25
1+2sinxcosx = 9/25
sinxcosx = -8/25
Sum of root sinx + cos x = -p/2 = 3/5
Therefore, p = -6/5
Product of root sinxcosx = q/2 = -8/25
Therefore, q = -16/25
p^2-8q
= (-6/5)^2-8(-16/25)
=36/25 - 128/25
=-92/25

用product root同 sum of root做...求出p and q 值...就能求出p^2-8q之值

2008-04-29 22:30:18 補充：
同咪a^2+b^2=(a+b)^2-2ab
sin^2x+cos^2x=1