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Add & subtract polynomials: find the error

Isatou tried to subtract two polynomials. Her work is shown below:
=(3c25cd+d2)(7c2+4cd9d2)=3c25cd+d27c2+4cd9d2Step 1=(37)c2+(5+4)cd+(19)d2Step 2=4c2cd8d2Step 3\begin{aligned} &\phantom{=}(3c^2-5cd+d^2)-(7c^2+4cd-9d^2) \\\\ &=3c^2-5cd+d^2-7c^2+4cd-9d^2&\text{Step }1 \\\\ &=(3-7)c^2+(-5+4)cd+(1-9)d^2&\text{Step }2 \\\\ &=-4c^2-cd-8d^2&\text{Step }3 \end{aligned}
At which step did Isatou make an error, if at all?
Choose 1 answer:
Choose 1 answer: