Problem 15 (trigonometry): Always observe the answer options before you start to work on a problem. For example, if the answer options show bounds, then don’t go ahead and try to calculate the actual value.
Problem 16 (circles): Descartes’ Circle Theorem states that $2(a^2 + b^2 + c^2 + d^2)=(a+b+c+d)^2$, where $a$, $b$, $c$, and $d$ represent the curvatures of the circles. (If two circles are internally tangent, the circle surrounding the other one will have a negative curvature representing it).
Problem 18 (number theory): Modular arithmetic can be a helpful tool with AMC.
Problem 20 (logarithms): For logarithms always consider special cases such as negative values, 0, or 1.
Problem 21 (algebra): Construct cases with purpose and not randomly. Also, remember the Rational Root Theorem for problems.
Problem 23 (algebra): If Vieta’s is confusing for a problem, try another method too.