Question:
why cos(90 x)= - sinx
Answer:
We know that
cos(x+y) = cosx*cosy - sinx*siny
Now
cos(90+x) = cos90*cosx - sin90*sinx
= 0*cosx - 1*sinx (since cos90 =0 and sin 90 =1)
= -sinx
So cos(90+x) = -sinx
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