Difference between revisions of "2019 AIME I Problems/Problem 8"

Revision as of 19:25, 14 March 2019

The 2019 AIME I takes place on March 13, 2019.

NOT ALLOWED!

Can't do that!

@@ Line 5: / Line 5: @@
 NOT ALLOWED!
-==Solution 2 (Another BASH)==
+==Solution 2==
-First, for simplicity, let <math>a=\sin{x}</math> and <math>b=\cos{x}</math>. Note that <math>a^2+b^2=1</math>. We then bash the rest of the problem out. Take the tenth power of this expression and get <math>a^{10}+b^{10}+5a^2b^2(a^6+b^6)+10a^4b^4(a^2+b^2)=\frac{11}{36}+5a^2b^2(a^6+b^6)+10a^4b^4=1</math>. Note that we also have <math>\frac{11}{36}=a^{10}+b^{10}=(a^{10}+b^{10})(a^2+b^2)=a^{12}+b^{12}+a^2b^2(a^8+b^8)</math>. So, it suffices to compute <math>a^2b^2(a^8+b^8)</math>. Let <math>y=a^2b^2</math>. We have from cubing <math>a^2+b^2=1</math> that <math>a^6+b^6+3a^2b^2(a^2+b^2)=1</math> or <math>a^6+b^6=1-3y</math>. Next, using <math>\frac{11}{36}+5a^2b^2(a^6+b^6)+10a^4b^4=1</math>, we get <math>a^2b^2(a^6+b^6)+2a^4b^4=\frac{5}{36}</math> or <math>y(1-3y)+2y^2=y-y^2=\frac{5}{36}</math>. Solving gives <math>y=1</math> or <math>y=\frac{1}{6}</math>. Clearly <math>y=1</math> is extraneous, so <math>y=\frac{1}{6}</math>. Now note that <math>a^4+b^4=(a^2+b^2)-2a^2b^2=\frac{2}{3}</math>, and <math>a^8+b^8=(a^4+b^4)^2-2a^4b^4=\frac{4}{9}-\frac{1}{18}=\frac{7}{18}</math>. Thus we finally get <math>a^{12}+b^{12}=\frac{11}{36}-\frac{7}{18}*\frac{1}{6}=\frac{13}{54}</math>, giving <math>\boxed{067}</math>.
+Can't do that!
-- Emathmaster
 ==See Also==
 {{AIME box|year=2019|n=I|num-b=7|num-a=9}}
 {{MAA Notice}}

2019 AIME I (Problems • Answer Key • Resources)
Preceded by Problem 7	Followed by Problem 9
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All AIME Problems and Solutions