Question

如果a²-a-1=0,那么5(a+3)(a-4)=__________

quickly,快速点儿

Answer 1

a2-a-1=0
所以a2-a=1
原式=5（a2-a-12)
=5(a2-a)-60
=5*1-60
=-55

Answer 2

5(a+3)(a-4)=5(a^2-a-12)=5[(a^2-a-1)-11]=5([(a^2-a-1)-5*11=5*0-5*11=-55