Question 1091112
C)

{{{cos(theta) = sin(theta+pi/2)}}}

This is an identity you should memorize.  Cosine leads Sine by 90 degrees (pi/2 radians).  
{{{cos(0) = sin(pi/2) = 1}}}
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{{{cos(pi/2) = sin(pi) = 0}}}
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{{{cos(pi) = sin(3pi/2) = -1 }}}
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{{{cos(3pi/2)= sin(2pi) = 0 }}}
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etc.