T N N l\ﬂ8$in 40° — cos 70° = /3 .cos80° – InterviewSolution

1.

T N N l\ﬂ8$in 40° — cos 70° = /3 .cos80°

Answer»

LHS =sin40-cos70

=sin40-cos(90-20)

=sin40-sin20

now use formulasinA-sinB=2cos(A+B)/2.sin(A-B)/2

hence.sin40-sin20=2cos30.sin10

=2 x √3/2 x cos80

=√3cos80° =RHS

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