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Answer: C) 0, 90</h3>
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sin(x) + cos(x) = 1
( sin(x)+cos(x) )^2 = 1^2
sin^2(x) + 2sin(x)cos(x) + cos^2(x) = 1
sin^2(x) + cos^2(x) + 2sin(x)cos(x) = 1
1 + 2sin(x)cos(x) = 1
2sin(x)cos(x) = 0
sin(x) = 0 or cos(x) = 0
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If sin(x) = 0, then x = 0 or x = 180
However, only x = 0 works while x = 180 is extraneous (plug x = 180 into the original equation and you should get -1 on the LHS). So we ignore x = 180.
If cos(x) = 0, then x = 90 or x = 270
x = 90 works while x = 270 is extraneous. We ignore x = 270.
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In summary, only x = 0 and x = 90 are solutions.
