If `sin(x+20) = cos(x-10)` find the value of x.
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we know the following rule from trigonometry.
`sin(90-theta) = costheta`
`x+20 = 90-(70-x)`
`sin(x+20) = sin(90-(70-x)) = cos(70-x)`
`sin(x+20) = cos(x-10)`
`cos(70-x) = cos(x-10)`
Since all the answers are less than of equal to 90 we can say;
`70-x = x-10`
`x = 40`
So the answer is x = 40. Correct answer is d)
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