Respuesta :

Answer:

Step-by-step explanation:

(a+b)²=a²+2ab+b²

9x²+30x+ca

=(3x)²+2×3x×5+c

=(3x)²+2×3x×5+5²-5²+c

=(3x+5)²-25+c

it is a perfect square if -25+c=0

or c=25

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