Solution 3.1:1f
From Förberedande kurs i matematik 2
Let's begin by calculating some powers of i,
| \displaystyle \begin{align}
i^2 &= i\cdot i = -1\,,\\[5pt] i^3 &= i^2\cdot i = (-1)\cdot i = -i\,,\\[5pt] i^4 &= i^2\cdot i^2 = (-1)\cdot (-1) = 1\,\textrm{.} \end{align} |
Now, we observe that because \displaystyle i^4=1, we can try to factorize \displaystyle i^{11} and \displaystyle i^{20} in terms of \displaystyle i^4,
| \displaystyle \begin{align}
i^{11} &= i^{4+4+3} = i^4\cdot i^4\cdot i^3 = 1\cdot 1 \cdot (-i) = -i\,,\\[5pt] i^{20} &= i^{4+4+4+4+4} = i^4\cdot i^4\cdot i^4\cdot i^4\cdot i^4 = 1\cdot 1 \cdot 1\cdot 1 \cdot 1 = 1\,\textrm{.} \end{align} |
The answer becomes
| \displaystyle i^{20}+i^{11}=1-i\,\textrm{.} |
