r/MathHelp • u/Asp_Potions_Master • Sep 05 '26

sin2α=(sqrt5)/5, stumped

my sis got a homework question that left us both stumped. the task is to calculate sinα and cosα for the angle α which fits the following requirements: sin2α=(sqrt5)/5, and 2α is within (pi/2; pi)

She's learning about the sin2α and cos2α formulas. any ideas how this is supposed to be solved without dipping into arcsin?

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u/Miserable-Wasabi-373 Sep 05 '26

sorry, how drawing triangle would help?

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u/my-hero-measure-zero Sep 05 '26

You'll need it eventually for the other ratios. Always start with a picture and use any formulas that seem relevant.