Bài 60 trang 219 SGK Đại số 10 Nâng cao

Chọn đáp án đúng:

Nếu (sin alpha  + cos alpha  = {1 over 2}) thì sin2α  bằng: 

(eqalign{
& (A),{3 over 8},,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,(B), - {3 over 4} cr
& (C),{1 over {sqrt 2 }},,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,(D),{3 over 4} cr} )

Đáp án

Ta có:

(eqalign{
& sin alpha + cos alpha = {1 over 2} Rightarrow 1 + 2sin alpha cos alpha = {1 over 4} cr
& Rightarrow sin 2alpha = - {3 over 4} cr} ) 

Chọn (B)

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