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Question

The value of sin22° + sin24° + sin26° + ... + sin290° is ____________.

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Solution


sin22° + sin24° + sin26° + ... + sin290°

= sin22° + sin24° + sin26° + ... + sin284° + sin286° + sin288°+ sin290°

= [(sin22° + sin288°) + (sin24° + sin286°) + (sin26° + sin284°) + .... 22 pair of terms] + sin290°

= [(sin22° + cos22°) + (sin24° + cos24°) + (sin26° + cos26°) + .... 22 pair of terms] + sin290° [sin(90º − θ) = cosθ]

= (1 + 1 + 1 + ... 22 terms) + 1 [sin2θ + cos2θ = 1 and sin90° = 1]

= 22 + 1

= 23

The value of sin22° + sin24° + sin26° + ... + sin290° is ______23______.

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