21 Use the sine rule or the cosine rule
a

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Answer

Substitute $\sin A$ with $\sin x$ to get $\sin x = \boxed{\frac{a}{b}}$

Steps

Step 1 ：Construct a right triangle with sides $a$, $b$, and $c$ such that $\angle A = x$, $\angle B = 90^\circ$, and $\angle C = 180^\circ - 90^\circ - x$

Step 2 ：By the sine rule, we have $\frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C}$

Step 3 ：Since $\angle B = 90^\circ$, we have $\sin B = 1$

Step 4 ：Thus, $\frac{a}{\sin A} = b$

Step 5 ：Rearrange to get $\sin A = \frac{a}{b}$

Step 6 ：Substitute $\sin A$ with $\sin x$ to get $\sin x = \boxed{\frac{a}{b}}$

21 Use the sine rule or the cosine rulea

Answer

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21 Use the sine rule or the cosine rule
a