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6. D
a*b = 1*2 + -1*y = 2 - y = 1
y = 1
7. B
AB*a = (2, 3) * (2k-1, 2) = 4k-2 + 6 = 0
k = -1
8. C
9. C
sina^2 = 1 - cosa^2 = 144/169
a 在第四象限, sina = -12/13
cos(a-PI/4) = cosacos(PI/4) + sinasin(PI/4) = sqrt(2)/2 (cosa + sina) = -7sqrt(2)/26
10. B
1+cos2a = sina^2 + cosa^2 + cosa^2 - sina^2 = 2cosa^2
a*b = 1*2 + -1*y = 2 - y = 1
y = 1
7. B
AB*a = (2, 3) * (2k-1, 2) = 4k-2 + 6 = 0
k = -1
8. C
9. C
sina^2 = 1 - cosa^2 = 144/169
a 在第四象限, sina = -12/13
cos(a-PI/4) = cosacos(PI/4) + sinasin(PI/4) = sqrt(2)/2 (cosa + sina) = -7sqrt(2)/26
10. B
1+cos2a = sina^2 + cosa^2 + cosa^2 - sina^2 = 2cosa^2
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