(1g2)的二次方+1g20乘以1g5

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(1g2)的二次方+1g20乘以1g5

(1g2)的二次方+1g20乘以1g5
(1g2)的二次方+1g20乘以1g5

(1g2)的二次方+1g20乘以1g5
(1g2)^2 + 1g20 X 1g5
=(1g2)^2 +(lg10+lg2)(lg10-lg2)
=(1g2)^2 +(1+lg2)(1-lg2)
=(1g2)^2 + 1 -(1g2)^2
=1

1g20=1g(4*5)=1g4+1g5=21g2+1g5
带入原式=(1g2)^2+21g2*1g5+(1g5)^2
这是完全平方公式,所以=(1g2+1g5)^2=(1g10)^2=1