Two diagonals

The rhombus has a side length of 12 cm and a length of one diagonal of 21 cm. What is the length of the second diagonal?

Correct answer:

u₂ =  11.619 cm

Step-by-step explanation:

$$\begin{gathered} a=12 \\ {u}_1=21 \\ {a}^2=(u_1\mathrm{/}2{)}^2+(u_2\mathrm{/}2{)}^2 \\ {u}_2=2\cdot \sqrt{a^2-(u_1\mathrm{/}2{)}^2}=2\cdot \sqrt{12^2-(21\mathrm{/}2{)}^2}=3\sqrt{15}=11.619\text{ cm} \end{gathered}$$

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