$a)$
Ta có :
`\hat{xOy} + \hat{yOz} = 180^o` ( hai góc kề bù )
`=> 70^o + \hat{yOz} = 180^o`
`=> \hat{yOz} = 110^o`
$b)$
Ta có :
`\hat{yOz} + \hat{xOz} = 180^o` ( hai góc kề bù )
`=> 110^o + \hat{xOz} = 180^o`
`=> \hat{xOz} = 70^o`
Ta có :
`\hat{xOz} = \hat{xOt} + \hat{tOz}`
`=> 70^o = 40^o + \hat{tOz}`
`=> \hat{tOz} = 30^o`
`\hat{yOt} = \hat{yOz} + \hat{zOt}`
`\hat{yOt} = 110^o + 30^o`
`\hat{yOt} = 140^o`