The Brainliest Answer!
Since a∝b & a∝c ,

we haven't multiplied both we actually combined both equation , if we combine both equations since a is directly proportional to both we can write it as

a∝ bc ,

like u can take the example of gravitational force , which says

F∝ 1/r²

F ∝ m1m2

where m1 & m2 are masses of two different objects

& we get the formula by combining

F ∝ m1m2/r²

Hope it clears .... ^_^
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