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 8 $f(550)$ if $f(11)=1$ and $f(a)f(b)=f(a+b)+f(a-b)$ 7 Is the sum of all rational numbers between two integers infinity 6 How to find the limit of $\frac{\sqrt{x^2+9}-3}{x^2}$? as x approaches 0 6 How many 5-letter words from ABRACADABRA 5 how many ways of arranging given 7 two digit positive integers so that the sum of every four consecutive integer is divisible by 3?

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 2 Prove: if $Z_1$ and $Z_2$ are complex, then $Z_1*Z_2=R_1R_2[\cos(\theta_1+\theta_2)+i\sin(\theta_1+\theta_2)]$ 1 How to derive two angles in an ambiguous case 0 If I roll three dice at the same time, how many ways the sides can sum up to $13$? 0 What is cos and sin ACTUALLY doing?

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 3 algebra-precalculus × 90 0 calculus × 61 3 trigonometry × 36 0 combinatorics × 15 2 proof-verification × 14 0 integration × 14 2 complex-numbers × 10 0 limits × 13 1 geometry × 9 0 derivatives × 11

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