Question

求这道数学题解析~谢谢大佬！！

求这道数学题解析~谢谢大佬！！以上！

Answer 1

sin(π/4 + α)=sin(π/2 - π/4 + α)
=sin[π/2 - (π/4 - α)]
=cos(π/4 - α)=1/3
∴cos(π/2 - 2α)=cos[2(π/4 - α)]
=2cos²(π/4 - α) - 1
=2•(1/3)² - 1
=2/9 - 1
=-7/9

Answer 2

已知sin(π/4+a)=1/3 求cos(π/2-2a)
sin(π/4)cosa+cos(π/4)sina=1/3
(√2/2)(sina+cosa)=1/3
(1/2)(sina+cosa)²=1/9
sin²a+2sinacosa+cos²a=2/9
sin2a=-7/9
cos(π/2-2a)=sin2a=-7/9

Answer 3

太积极押题卷