49[m+n]^2-[3m-n]^2

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49[m+n]^2-[3m-n]^2

49[m+n]^2-[3m-n]^2
49[m+n]^2-[3m-n]^2

49[m+n]^2-[3m-n]^2
49(m+n)^2-(3m-n)^2
=[7(m+n)]^2-(3m-n)^2
=[7(m+n)+(3m-n)][7(m+n)-(3m-n)]
=(7m+7n+3m-n)(7m+7n-3m+n)
=(10m+6n)(4m+8n)
=8(5m+3n)(m+2n)

49[m+n]^2-[3m-n]^2=(7m+7n+3m-n)(7m+7n-3m+n)=(10m+6n)(4m+8n)=8(5m+3n)(m+2n)

把7[m+n]看做a,把3m-n看做b,然后利用平方差公式=8(5m+3m)(m-2n)